Syringe with incrementally actuated plunger

The present invention relates to a syringe for delivering only a single dose of a medicament at each actuation of the syringe. The device includes a cylinder which is adapted to contain the medicament, the cylinder having a piston which is journalled in the bore of the cylinder for axial movement in the bore towards an outlet. The piston can be moved axially within the bore of the cylinder by means of a plunger member which carries one or more drive teeth which are adapted to engage teeth carried by an axially moveable drive member. The axial spacing between the axially adjacent teeth on either the plunger member or the drive member corresponds to the forward axial movement of the plunger member which is required to discharge a single dose of medicament. The drive member can also be retracted axially only a predetermined distance which corresponds to one of the tooth spacings with respect to the plunger.

The present invention relates to a device, notably to a single dose 
syringe. 
BACKGROUND TO THE INVENTION 
Medicaments are often administered to a patient by means of a syringe in 
which the user depresses the exposed end of a plunger journalled axially 
in the syringe body to drive a piston journalled within the bore of the 
syringe body a given axial distance to eject a quantity of fluid from the 
syringe body through a needle mounted at the distal end of the syringe 
body, ie. that end removed from the end at which the user acts upon the 
exposed end of the plunger shaft. The amount of fluid ejected will depend 
upon the extent of movement of the plunger and this is typically assessed 
by the user monitoring a number of clicks or other audible signals 
generated as the plunger is moved. Alternatively, the user can monitor the 
extent movement against a fixed scale or the like. 
Where a single dose is to be administered, the user often draws sufficient 
fluid into the barrel of the syringe from a bottle or the like by 
retracting the plunger to draw fluid into the body of the syringe through 
the needle and ejects the whole of that dose by a single depression of the 
plunger. However, this is inconvenient where repeated doses of the same 
medicament are to be administered. It is therefore customary for a syringe 
to incorporate a cartridge or other reservoir so that a user can 
administer a number of dosages without the need to replace the source of 
the medicament after each dosage. The syringe mechanism is therefore 
designed to permit repeated actuation of the plunger and means must be 
provided for enabling the plunger to be moved the desired amount for each 
dose and, usually, for the user to select the dosage to be administered. 
However, this carries the risk that the user may administer an over or 
under dose by incorrect selection of the amount of medicament discharged 
from the cartridge or reservoir, and/or by repeated partial actuation of 
the device, for example where the user has a tremulous hand. 
Various forms of dosage selection mechanism have been proposed, see for 
example European Patent Application No 0037696 A1 and British Patent 
Application No 2109690 A. However, such mechanisms rely upon the accurate 
selection of the dosage by the user and accurate operation of the device 
if consistent dosages are to be achieved. 
It has been proposed in, for example British Specification No 1230522, to 
form a number of tangentially and axially off set radial projections on 
the shaft of the plunger, the axial off set of each of which corresponds 
to the axial movement of the plunger required to administer a selected 
dose. The proximal end of the syringe body carries a transverse plate 
through which the plunger shaft passes. The plunger can be moved axially 
until the distal end of one of the projections engages the face of the 
transverse plate and thus halts further axial movement of the plunger. The 
plunger must then be rotated about its longitudinal axis to bring the 
projection into alignment with a radial slot in the plate which allows the 
projection to pass axially through the plate and thus allow further 
movement of the plunger until the next projection engages the face of the 
plate. Such a device provides means for discharging single doses from the 
syringe, However, the user can administer partial doses by incomplete 
depression of the plunger and can retract the plunger during use, for 
example by operating the plunger tremulously, to administer an unknown 
dose of medicament. 
It has been proposed in European Patent Application No 0225439 A1 to 
provide the plunger of a syringe with an axial series of radially 
projecting teeth on each side thereof and to pass the plunger shaft 
through a transversely moveable block member which carries a second series 
of teeth adapted to engage with the teeth on the plunger. The block member 
is urged radially by a spring so that the teeth on the block engage those 
on the plunger and thus prevent axial movement of the plunger. When the 
block member is depressed against the spring, the engagement between the 
teeth is broken to allow the plunger to move axially. In order to limit 
the forward movement of the plunger, there is provided a spring loaded arm 
pivoting about a transverse axis, the arm carrying a third set of axial 
teeth which partially engages with the teeth on the top face of the 
plunger. By virtue of the arc of swing of the arm, the teeth it carries 
progressively engage with the teeth on the plunger as the plunger moves 
axially forward, until the teeth are fully engaged and prevent further 
axial travel of the plunger. 
Such a mechanism is said to restrict forward movement of the plunger to a 
series of pre-set incremental movements. However, the mechanism requires 
concerted movement of the block and arm members to alternately secure and 
release the plunger. Furthermore, the plunger can be withdrawn an 
indeterminate distance when the teeth of the block member are disengaged 
from the teeth on the plunger. A user therefore does not know how much 
potential forward movement of the plunger is available in the device and 
hence the dosage which will be administered. 
We have devised a form of syringe operative mechanism which reduces the 
above problems. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a device for the administration 
of a medicament by means of a needle adapted to be inserted into the body 
of a person, which device comprises a cylinder adapted to contain the 
medicament and having a piston journalled in the bore of the cylinder for 
axial movement in the bore towards an outlet provided with the needle or 
adapted to receive the needle in mounting engagement therewith, the piston 
being adapted to be moved axially within the bore of the cylinder by means 
of a plunger member, which plunger member carries one or more drive teeth 
adapted to engage with teeth carried by an axially moveable drive member 
adapted to be acted upon by the user, characterised in that: 
a. the axial spacing between axially adjacent teeth on one of the plunger 
member or the drive member corresponds to the forward axial movement of 
the plunger member required to discharge a single dose of the medicament; 
and 
b. means are provided for retracting the drive member axially a 
predetermined distance corresponding to one said tooth spacing with 
respect to the plunger whereby forward axial movement of the drive member 
with the teeth of the drive and plunger members engaged is adapted to 
administer said single dosage of the medicament. 
By providing the spacing between the teeth on the plunger or the drive 
first member at the equivalent of one dosage axial movement and limiting 
the retraction of the other member with respect to the first member to the 
axial distance corresponding to a single dose travel, the device can 
deliver only a single dose of medicament per actuation of the device. The 
device of the invention thus avoids the need to provide dosage selection 
means with the attendant risk that the user may select an incorrect dose. 
For convenience, the invention will be described hereinafter in terms of 
the plunger carrying drive teeth which have the required single dose 
travel spacing and the drive member being the member whose axial extent of 
retraction is limited to that single dose travel. 
Furthermore, due to the spacing of the teeth on the plunger member and the 
limit on the extent of movement of the drive member, the teeth on the 
drive member cannot engage the next axial location upon the plunger until 
the plunger has moved axially for it full extent of travel for the dose 
being discharged, since the next teeth on the plunger to be engaged by the 
drive member will not be fully exposed for engagement by the drive member 
until the plunger has completed its forward stroke. If the drive between 
the drive member and the plunger member of the device of the invention is 
inadvertently disengaged before the forward stroke of the plunger has been 
completed, the teeth of the drive member and the plunger will be out of 
register at all permitted positions of travel of the drive member and will 
not re-mesh with one another, except at the exact point where 
disengagement took place. It is necessary to relocate the drive member 
exactly with respect to the plunger member to enable the teeth on the 
drive member to mesh with the teeth on the plunger member and thus allow 
the drive to be re-engaged. Thus, the user can discharge only a complete 
dose, even where the drive is accidentally disengaged during operation and 
cannot re-engage the drive at a position which would allow excessive or 
insufficient medicament to be discharged. 
Preferably, means are provided whereby the drive member cannot be 
disengaged from the plunger member intermediate the extremes of travel of 
the drive member during the administration of a complete dose and 
re-engagement of the drive and plunger members can only be achieved at the 
rearward extreme of the travel of the drive member. The device therefore 
cannot be operated to deliver anything but a single full dose of the 
medicament and a user has to complete the delivery stroke before the drive 
member can be retracted to prepare the device for delivery of a subsequent 
dose. 
Preferably, the drive member is spring or otherwise biassed to its axially 
fully retracted position at which its teeth can mesh with the teeth of the 
plunger member at the start position for the forward movement of the 
plunger member for the administration of each dose of medicament. 
Preferably, the engagement and disengagement of the drive member teeth from 
the drive teeth of the plunger member require a positive action, eg. 
relative rotation between the drive member and the plunger member, by the 
user at the forward end of the plunger stroke and the rearward end of the 
drive member stroke. Such positive disengagement of the drive between the 
drive and plunger members permits relative axial movement between the 
plunger member and the drive member whilst the drive member is retracted 
and the plunger member remains stationary. This substantially prevents 
inadvertent operation of the device to administer multiple dosages. 
Preferably, the plunger member is prevented from axial retraction by 
ratchet or other means acting upon the plunger member whereby the plunger 
is prevented from being retracted as the drive member is retracted, thus 
achieving uni-directional drive of the plunger by the drive member. 
The device of the invention comprises a body portion which can be of any 
suitable shape and design and which houses the plunger and drive 
mechanism. The invention can be applied to syringes in which a plurality 
of doses of the medicament are held within the bore of the syringe body 
which forms the cylinder containing the piston against which the plunger 
member acts and from which the doses of medicament are discharged by 
progressive movement of the piston axially in the bore. However, the 
invention is of especial application where the medicament is held in a 
replaceable cylindrical cartridge attached to the distal end of the 
syringe body, the cartridge having a piston journalled in the bore of the 
cartridge. 
For convenience, the invention will be described hereinafter in terms of a 
syringe body carrying a replaceable medicament cartridge at the distal end 
of the syringe body. 
Preferably, the syringe body portion is similar to a conventional syringe 
body which is adapted to receive a medicament cartridge at the distal end 
of the body. The cartridge provides the cylinder and piston, the body of 
the syringe providing an axial housing which carries the drive member and 
the plunger member which acts directly or through a member operatively 
associated therewith on the piston in the cartridge. The assembled 
cartridge and housing thus have a generally cylindrical configuration with 
radial symmetry about the longitudinal axis of the device. The cartridge 
can be of any suitable type and many are available commercially and will 
usually be secured to the syringe body by a bayonet, screw or other fit of 
the cartridge into a recess at the distal end of the syringe body. 
Preferably, the cartridge is secured to the distal end of the syringe body 
by means of a generally cylindrical holder within which the cartridge is 
secured, for example the cartridge is a push fit within the holder. 
Alternatively, the cartridge is secured within the holder by means of 
snapfit ribs or the like which engage the rim below the crimped end cap 
usually present in commercially available medicament cartridges. Since the 
holder can usually be made to closer dimensional tolerances than the glass 
body of the cartridge, location of the distal end of the cartridge 
accurately within the holder provides an accurate positioning of the 
distal end of the cartridge with respect to the travel of the plunger when 
the holder is mounted on the syringe body. It is also preferred that the 
holder be secured to the syringe body by means of a screw thread so that 
minor inaccuracies in the fabrication of the device and the holder can be 
accommodated. 
Preferably, the rearward rim of the cartridge or the holder engages radial 
lugs or the like carried within the recess in the syringe body into which 
the cartridge/holder locates. The lugs act to cause ratchet teeth to 
engage with teeth on the shaft of the plunger so as to prevent rearward 
movement, ie. retraction, of the plunger once the cartridge/holder has 
been secured in position. In a particularly preferred embodiment, the lugs 
take the form of two or more axially extending arms located in the base of 
the recess and having a tapered radially outward faces. The arms carry the 
ratchet teeth upon their radially inward faces. As the rim of the 
cartridge or holder rides axially rearward along the arms as the 
cartridge/holder is inserted into the recess, it flexes the arms inwardly 
and brings the ratchet teeth into engagement with the teeth on the plunger 
member. When the cartridge/holder is mounted fully home upon the syringe 
body, the ratchet teeth prevent retraction of the plunger so that the 
plunger remains static when the drive member is retracted between 
actuations of the device. However, as the cartridge/holder is removed from 
the syringe body, the ratchet teeth are disengaged from the plunger, for 
example the arms carrying the ratchet teeth are allowed to flex outwardly 
and to carry the ratchet teeth clear of the teeth on the plunger. When the 
ratchet teeth are disengaged from the plunger and the cartridge/holder 
removed to expose the distal end of the plunger, the plunger can be 
extended to engage the piston within a replacement cartridge/holder as it 
is offered up to the body of the syringe. The plunger is then free to 
retract as the cartridge/holder is secured to the syringe until the 
ratchet teeth are engaged with the plunger as the cartridge/holder rim 
seats home in the recess. The distal end of the plunger is thus located 
against the piston and it is not necessary for a user to operate the 
device several times to advance the plunger into engagement with the 
piston when the device is first used. 
The teeth on the shaft of the plunger which engage the ratchet teeth are 
preferably a separate set of teeth from those, the drive teeth, which are 
to engage the teeth on the drive member, and are located adjacent the 
distal end of the plunger shaft. Preferably, this separate set of teeth 
have a finer axial spacing than the drive teeth on the plunger, so that 
engagement of the ratchet teeth with the separate teeth can occur over 
smaller axial intervals of the movement of the plunger. In order that the 
engagement of the various sets of teeth can be synchronised, the separate 
teeth have an axial spacing which is an integer sub-multiple of the axial 
spacing of the drive teeth on the plunger, typically one third or a 
quarter. 
The syringe body contains the plunger/drive mechanism which is used to 
advance the piston in the cartridge. Typically, the plunger and drive 
members are orientated upon the longitudinal axis of the syringe body and 
the drive member is provided with one or more teeth which are directed 
radially inwardly to engage the drive teeth on the plunger which are 
directed radially outward. However, the device may be configured with the 
teeth on the drive member directed outwardly to act on inwardly directed 
drive teeth carried by a sleeve acting as the plunger or a connection 
between the plunger and the drive members. For convenience, the invention 
will be described hereinafter in terms of a substantially solid plunger 
member carrying radially outwardly directed drive teeth which engage with 
radially inwardly directed teeth carried by the drive member, at least the 
distal end of which is in the form of a sleeve within which the plunger is 
journalled or in the form of a separate sleeve member forming the distal 
end of the drive member and operatively connected to the remainder of 
drive member. 
In the device of the present invention, the teeth on each of the drive and 
plunger members can be the same or different but are of forms which can 
mutually engage so as to provide a positive drive between the drive member 
and the plunger on the forward stroke of the drive member. However, it is 
preferred that the tooth on the drive and plunger members be of the same 
form so as to facilitate engagement and disengagement of the teeth. The 
teeth are preferably of a squared cross-section. However, they can have a 
ratchet or a saw tooth configuration so that the drive teeth on the 
plunger member present a rearwardly facing scarp face, typically at 
substantially 90.degree. to the longitudinal axis of the plunger member, 
with a forward inclined face which extends to the foot of the scarp face 
of the preceding tooth. However, the slope need not extend fully to the 
foot of the preceding tooth, but there may be an axially flat portion 
between successive teeth. The teeth on the drive member are similar but of 
the reversed configuration so that the rearward or scarp face of a plunger 
drive tooth is engaged by the forward or scarp face of a drive member 
tooth. If desired, the opposed faces can be inclined so that they 
lockingly engage. 
The axial pitch or spacing between the teeth is such that the plunger is 
driven forward axially by the drive member one plunger drive tooth space 
for each actuation of the device and this distance of travel corresponds 
to that required at the piston to discharge a single dose of medicament 
from the cartridge. Preferably, the plunger member carries an axial series 
of drive teeth spaced apart by the travel required for a single dose and 
the drive member carries one tooth to engage the drive teeth successively. 
As stated above the axial retraction of the drive member is limited to the 
axial spacing between adjacent drive teeth on the plunger member. 
Preferably, the drive member travels between axially spaced apart stop 
members so that its axial travel is limited in each direction and 
registration with the teeth of the plunger member at the rearward extend 
of its travel is facilitated. Preferably, the drive member is spring 
loaded to the axially rearward stop. In this way, once the drive member 
has completed its forward stroke and engages the forward stop member, the 
drive mechanism can be disengaged and the drive member automatically 
retracts to the axially rearward stop at which the tooth/teeth of the 
drive member will register with the next gap(s) between the drive teeth on 
the plunger member so that the drive can be re-engaged for the subsequent 
dose. 
The plunger and drive members are formed so that the drive member can be 
moved axially rearwardly with respect to the plunger member. As indicated 
above, it is preferred that the plunger member be journalled in an axial 
bore within the drive member and that the teeth of the drive member are 
carried at the distal end of the bore or of a separate sleeve member which 
forms the distal end of the drive member. Thus, the teeth of the drive 
member can be sprung, for example by splitting the side wall of the sleeve 
member or distal end of the drive member to form axial spring arms 
carrying the teeth, so that they ride over the drive teeth of the plunger 
member when the drive member is retracted axially but re-engage 
automatically when the drive member moves forward with respect to the 
plunger to provide a uni-directional drive between the drive and plunger 
members. 
However, it is preferred to form the plunger and/or the drive member so 
that some positive action by the user is required to disengage and 
re-engage the drive between the two members. Thus, the plunger can be 
formed with two opposed flatted faces, and the drive member carries its 
teeth along two diametrically opposed axial lines. Upon rotation of the 
plunger and drive members through 90.degree. with respect to one another 
about their longitudinal axes, the teeth on the drive member are brought 
into register with the flatted faces of the plunger and therefore no 
longer engage the drive teeth on the plunger member. Rotation through a 
further 90.degree. or, preferably, rotation back through 90.degree. 
re-engages the teeth on the drive and plunger members to re-establish the 
positive drive between the plunger and drive members. If desired, stops or 
other means can be provided to limit the extent and direction of relative 
rotation of the drive and plunger members. 
Relative rotation of the plunger and drive members can be achieved by 
providing the drive member with a radially protruding pin or the like 
which engages in a guide slot or track in a portion of the syringe body 
within which the drive member is journalled, which portion can be rotated 
relative to the remainder of the syringe body to bring the teeth on the 
drive member out of and into engagement with the drive teeth on the 
plunger member at the forward and rearward extremities respectively of the 
axial travel of the drive member. Alternatively, the drive member, or a 
member operatively associated therewith, can extend rearwardly beyond the 
proximal limit of the syringe body to provide an exposed end cap which can 
be used both to move the drive member axially and to rotate the drive 
member. 
Alternatively, the distal end of the drive member can be provided with two 
or more axial arm members which carry the teeth to engage the drive teeth 
of the plunger member. The arm members can be carried by a separate 
connecting member or sleeve which operatively connects the remainder of 
the drive member and the plunger member. The arm members are adapted to 
flex or pivot radially so that the teeth carried by the arms can be 
brought into and out of engagement with the drive teeth on the plunger 
member by radial movement of the arms relative to the plunger member. 
Preferably, the arms are moved radially by a suitable cam mechanism 
carried internally by a rotatable sleeve member of the syringe body so 
that the teeth on the arms are brought radially into engagement with the 
drive teeth of the plunger member to engage the drive, but are allowed to 
splay when the drive is to be disengaged from the plunger. Suitable forms 
of such a radially acting clutch mechanism are described and claimed in 
our European Patent No 0037696 and our European Patent Application No 
0295075 A. Other forms of disengageable drive or clutch mechanism may be 
used if desired. 
The plunger member is also preferably provided with means, for example a 
pawl mechanism or a Torridon drive mechanism, which engages the teeth on 
the plunger member or the shaft of the plunger member so to prevent axial 
retraction of the plunger as the drive member is retracted. As stated 
above, such a mechanism can be interlinked with the mounting of a 
medicament cartridge/holder upon the distal end of the syringe body so 
that it is engaged when the cartridge/holder is mounted upon the syringe 
body, but can be disengaged when the cartridge/ holder is removed so as to 
allow the plunger member to be at least partially retracted into the 
syringe body to permit a new cartridge/holder to be fitted. 
As stated above, the drive member is preferably biassed to retract to a 
rearward stop and travels on its forward stroke to engage a forward stop, 
so as to define the extent of its travel. It is preferred also to provide 
the rearward end of the plunger member with a radial stop member. For 
example, the proximal end of the plunger can be formed as a full diameter 
component rather than with opposed flatted faces, to provide an annular 
shoulder which advances as the plunger advances. This stop will eventually 
engage the drive member, for example the rearward faces of the teeth 
carried by the drive member. Thus, when the plunger member has travelled 
for the total of all its possible forward travel and all doses within the 
cartridge have been dispensed, the drive member is trapped between the 
stop at the proximal, ie. rearward, end of the plunger and the forward 
stop defining the forward extent of travel of the drive member so that the 
drive member is prevented from advance or retraction. The user is thus 
given a positive indication as to when the device cannot deliver more 
medicament and a new cartridge/holder is required. 
The relative dimensions of the components of the device of the invention 
can be varied over wide ranges to suit the volume of the dosage to be 
discharged and the size of the cartridge housing to be used; and the 
device can readily be made from plastic or other materials using known 
techniques.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A syringe of the invention comprises a cylindrical housing 1 having a 
distal end 2 into which a cartridge 3 can be mounted by means of a 
cartridge holder 4 which is a snap or screw fit into the distal end of the 
housing 1. Within the cartridge is journalled a piston 5 which is moved 
incrementally axially by a plunger 6. Plunger 6 can be part of the syringe 
body, with the piston 5 being part of the replaceable cartridge, or the 
piston 5 can be carried by the plunger 6 as an integral part of the 
syringe body. 
The distal end of the cartridge holder 4 and/or the cartridge 3 carry a 
capillary needle, not shown, for injecting medicament into the body of a 
user. 
Within body 1 is journalled an axially moveable drive sleeve member 10 
carrying a radially inwardly projecting shoulder which forms a single 
circumferential tooth 11 which engages with radially outwardly projecting 
drive teeth 12 carried by the plunger 6. The teeth can have the squared 
profile as shown in FIG. 1, or can have a saw tooth or other configuration 
which achieves a positive drive between the plunger 6 and the sleeve 10. 
The axial spacing between successive drive teeth 12 corresponds to the 
axial movement of plunger 6, and hence of piston 5, required to discharge 
a single dose of the medicament from the cartridge 3. 
The forward end of sleeve 10 butts against an annular shoulder 13 formed at 
the forward extreme of the travel of sleeve 10 so as to define the limit 
of travel for the sleeve. A similar stop, not shown, defines the limit of 
the rearward travel, or retraction, of the sleeve 10. A spring 14 is 
trapped between sleeve 10 and stop 13 or their equivalents so that sleeve 
10 is biassed towards its fully retracted position. The axial spacing 
between the forward and rearward stops is such that the tooth 11 on sleeve 
10 moves axially for the distance required to discharge a single dose of 
medicament from the cartridge 3 so that when the tooth 11 engages a tooth 
12 it will move the plunger 6 axially for this distance. When the sleeve 
10 buts against the forward stop 13, it can be disengaged from the drive 
tooth 12 as described below and sleeve 10 allowed to retract to engage the 
rearward stop. At this point, the axially forward face of tooth 11 will 
register with the axially rearward face of the next drive tooth 12 so that 
the drive between the drive member 10 and the plunger 6 can be reengaged. 
If the sleeve 10 has not been fully retracted, it will be appreciated that 
the teeth 11 and 12 will foul one another and the drive cannot be 
re-engaged. 
The drive teeth 12 carried by the plunger 6 have an interrupted arcuate 
shape as shown in FIGS. 2 and 3 which marries with radially inward 
projections 15 in the plan shape of the annular shoulder 13. As shown in 
FIG. 2, the plunger and its teeth 12 are orientated so that the 
interruptions in the drive teeth correspond to the inward projections 15 
of the shoulder 13 so that the plunger can move freely past shoulder 13, 
whereas the forward end of sleeve 10 cannot pass projections 15 which thus 
act as a forward stop to the travel of sleeve 10. 
Sleeve 10 extends rearwardly to a knurled push button end 20 projecting 
from the end of the housing 1. The teeth carried internally upon sleeve 10 
are circumferentially interrupted in the same manner as shoulder 13 so 
that they engage the drive teeth 12 on plunger 6 in one angular 
orientation but do not engage when sleeve 10 is rotated about its 
longitudinal axis, so as to provide a hit or miss type of clutch between 
the sleeve 10 and plunger 6. Thus, when button end 20 is rotated to bring 
the teeth 11 into register with the teeth 12 on plunger 6 the drive 
between member 10 and plunger 6 is engaged and axial forward movement of 
member 10 will advance plunger 6. However, when end 20 is rotated through 
90.degree. the teeth 11 are carried out of register with the teeth 12 and 
the drive is disengaged. Sleeve 10 can then be retracted under the bias of 
spring 14 until sleeve 10 (carrying the end 20) butts against its rearward 
travel limiting stop, as shown dotted in FIG. 4. The sleeve 10 has moved 
axially one drive tooth spacing with respect to the plunger 6 and 
contra-rotation of button end 20 brings the teeth 11 on the sleeve into 
register with the teeth 12 on the plunger so as to re-engage the drive 
between the sleeve and the plunger. Button 20 can now be depressed as 
shown in FIG. 5 to return sleeve 10 forwardly to butt against stop 13. 
This drives piston 5 one dose axially along the bore of the cylinder of 
cartridge 3. As shown in FIG. 6, the plunger 6 can carry numbers or other 
markings 21 to register with a window or clear portion 22 of housing 1 to 
display the number of doses administered or remaining to be administered 
from the cartridge 3. 
It will be appreciated that the plunger and member 10 can carry more that 
two diametrically opposed sets of teeth as shown, for example the teeth 
could be arranged as three axial rows of teeth on each of the plunger and 
drive member at 120.degree. spacings rather than 180.degree. spacings. 
When the drive member has been moved to its axially forward extreme of 
travel, the device will be in the position shown in FIG. 1 and can not 
administer another dose until sleeve 10 has been disengaged from plunger 
6, retracted to its full extent and the drive with plunger 6 re-engaged as 
described above. 
Preferably, the device is provided with means whereby the drive between the 
sleeve 10 and plunger 6 cannot be disengaged during the forward travel of 
the sleeve and plunger. Thus, for example, the sleeve 10 carries a 
radially protruding pin which engages in an axial groove or guide in the 
wall of the housing, which has a 90.degree. circumferential cross portion 
at each end 
The sleeve, plunger, pin and guide are configured so that the teeth carried 
by the sleeve engage with the drive teeth carried by the plunger when the 
sleeve is aligned with the pin in the axial portion of the guide, but are 
disengaged when the pin is at the extreme of the circumferential cross 
portions of the guide. When the pin is carried out of engagement with the 
cross portion of the guide as the relative rotation of the sleeve and 
plunger is completed at the forward extreme of travel of the sleeve, the 
sleeve is released from the guide and can retract freely under the 
influence of the spring to align the pin axially with the entry to the 
axially rearward cross portion of the guide. In this way the positive 
drive between the sleeve and plunger can only be engaged or disengaged at 
either end of the full travel of the pin in the axial slot, the sleeve can 
retract with respect to the plunger only when the drive is disengaged at 
the forward extreme of travel of the sleeve, and the drive re-engaged only 
at the rearward extreme of travel of the sleeve. 
Preferably, the plunger 6 is provided with means which prevent axial 
retraction of the plunger when the sleeve 10 is retracted. As described 
above this can be achieved by providing disengagable ratchet teeth at the 
distal end of the syringe body which are actuated by the mounting of the 
cartridge/holder upon the syringe body and which engage with the drive 
teeth 12 on the plunger. However, such teeth 12 are usually comparatively 
widely spaced axially and it is preferred, as shown in FIG. 7, that the 
plunger can carry a secondary set of finer pitched teeth 30 which engage 
with a non-retraction ratchet mechanism 31 so that plunger 6 can not be 
retracted when sleeve 10 is retracted, thus preserving the one dose 
relative axial movement requirement before the drive can be re-engaged. 
The invention has been described above in terms of the sleeve 10 and the 
plunger 6 carrying teeth where the teeth have a single dose spacing. 
However, it is within the scope of the present invention to utilise more 
than one set of teeth on either or both of the sleeve and plunger, 
provided that the effect of the intermeshing of those sets of teeth is to 
ensure that the plunger moves only one dose distance axially upon each 
actuation of the device and that the drive between the sleeve and plunger 
cannot readily be disengaged and re-engaged except at each end of the 
axially travel of the sleeve. It will also be appreciated that the sleeve 
and plunger can be interchanged so that the button end 20 acts upon the 
plunger which then acts upon the sleeve which engages the piston 5; and 
that the plunger member can carry a single tooth which is engaged by 
successive teeth carried by the drive member which are spaced apart by the 
single dose travel distance. If desired, the drive member 10 can be spring 
or otherwise biassed to the forward end of its travel and the proximal end 
of the syringe housing can be provided with a suitable cam or other 
profile against which the end cap 20 bears so that rotation of the end cap 
initially disengages the drive between the drive member and the plunger, 
further rotation causes the end cap and hence the drive member to which it 
is linked to be moved axially rearwardly for the retracttion of the drive 
member with respect to the plunger member, further rotation re-engages the 
drive between the drive member and the plunger, and then releases the end 
cap for forward axial movement under the bias of the spring or other means 
to drive the plunger member forward and discharge the-dose of medicament.