A medical device comprising: - a base member, - a container comprising a container interior adapted to accommodate a first substance, and a container closure for fluidly sealing the container interior, - a variable volume reservoir comprising a reservoir interior adapted to accommodate a second substance, and a reservoir closure for fluidly sealing the reservoir interior - fluid connection means for establishing fluid communication between the container interior and the reservoir interior, - a cover removably mounted on a cover receiving portion to shield at least a portion of the variable volume reservoir, the cover being operatively coupled with the fluid connection means to cause a relative motion between the fluid connection means and at least one of the container closure and the reservoir closure in response to a relative motion between the cover and the base member, and - a cover engagement mechanism (9, 37, 46, 49) configured to prevent movement of the cover in a dismounting direction relative to the cover receiving portion when the cover and the base member are in a first relative position in which the container interior and the reservoir interior are fluidly unconnected and to allow movement of the cover in a dismounting direction relative to the cover receiving portion when the cover and the base member are in a second relative position in which fluid communication is established between the container interior and the reservoir interior, characterised by further comprising a blocking element movable with respect to the base member from a first position in which relative motion between the cover and the base member from the first relative position to the second relative position is prevented to a second position in which relative motion between the cover and the base member from the first relative position to the second relative position is allowed. - a base member, - a container comprising a container interior adapted to accommodate a first substance, and a container closure for fluidly sealing the container interior, - a variable volume reservoir comprising a reservoir interior adapted to accommodate a second substance, and a reservoir closure for fluidly sealing the reservoir interior - fluid connection means for establishing fluid communication between the container interior and the reservoir interior, - a cover removably mounted on a cover receiving portion to shield at least a portion of the variable volume reservoir, the cover being operatively coupled with the fluid connection means to cause a relative motion between the fluid connection means and at least one of the container closure and the reservoir closure in response to a relative motion between the cover and the base member, and - a cover engagement mechanism (9, 37, 46, 49) configured to prevent movement of the cover in a dismounting direction relative to the cover receiving portion when the cover and the base member are in a first relative position in which the container interior and the reservoir interior are fluidly unconnected and to allow movement of the cover in a dismounting direction relative to the cover receiving portion when the cover and the base member are in a second relative position in which fluid communication is established between the container interior and the reservoir interior,