A fixed dose injection device comprising the following in combination, a. a housing; and b. a dose setting and injecting assembly comprising, i. a rotatable gear having at least two teeth offset from each other at an angle α that corresponds to only one predetermined fixed dose of medicament, where the rotatable gear is fixed axially in the housing; ii. a plunger threadedly engaged with the rotatable gear having a pitch β that is matched with angle α so that the plunger moves distally during dose delivery a distance corresponding to the only one predetermined fixed dose; iii. a torsional spring, and a drive shaft wherein the torsional spring rotates the drive shaft back to a zero position if the drive shaft is rotated by an angle smaller than α during dose dialing such that dialing doses smaller than the only one predetermined fixed dose is prevented; iv. a trigger connected to the rotatable gear such that when the trigger is activated the torsional spring unwinds and causes the rotatable gear to rotate through angle α and to move the plunger distally to deliver the only one predetermined fixed dose; characterized in that v. a rotation stop that limits the rotation of the drive shaft, allowing a rotation beyond one tooth and preventing a rotation beyond more than one tooth of the rotatable gear such that setting a dose exceeding the only one predetermined fixed dose is prevented. a. a housing; and b. a dose setting and injecting assembly comprising, i. a rotatable gear having at least two teeth offset from each other at an angle α that corresponds to only one predetermined fixed dose of medicament, where the rotatable gear is fixed axially in the housing; ii. a plunger threadedly engaged with the rotatable gear having a pitch β that is matched with angle α so that the plunger moves distally during dose delivery a distance corresponding to the only one predetermined fixed dose; iii. a torsional spring, and a drive shaft wherein the torsional spring rotates the drive shaft back to a zero position if the drive shaft is rotated by an angle smaller than α during dose dialing such that dialing doses smaller than the only one predetermined fixed dose is prevented; iv. a trigger connected to the rotatable gear such that when the trigger is activated the torsional spring unwinds and causes the rotatable gear to rotate through angle α and to move the plunger distally to deliver the only one predetermined fixed dose; characterized in that v. a rotation stop that limits the rotation of the drive shaft, allowing a rotation beyond one tooth and preventing a rotation beyond more than one tooth of the rotatable gear such that setting a dose exceeding the only one predetermined fixed dose is prevented. i. a rotatable gear having at least two teeth offset from each other at an angle α that corresponds to only one predetermined fixed dose of medicament, where the rotatable gear is fixed axially in the housing; ii. a plunger threadedly engaged with the rotatable gear having a pitch β that is matched with angle α so that the plunger moves distally during dose delivery a distance corresponding to the only one predetermined fixed dose; iii. a torsional spring, and a drive shaft wherein the torsional spring rotates the drive shaft back to a zero position if the drive shaft is rotated by an angle smaller than α during dose dialing such that dialing doses smaller than the only one predetermined fixed dose is prevented; iv. a trigger connected to the rotatable gear such that when the trigger is activated the torsional spring unwinds and causes the rotatable gear to rotate through angle α and to move the plunger distally to deliver the only one predetermined fixed dose; characterized in that v. a rotation stop that limits the rotation of the drive shaft, allowing a rotation beyond one tooth and preventing a rotation beyond more than one tooth of the rotatable gear such that setting a dose exceeding the only one predetermined fixed dose is prevented.