Injection systems for drug delivery are disclosed. A system may include a container filled or fillable with a drug, a stopper movably disposed in the container, a plunger, an energy source outputting a force, a mechanical linkage operable to transmit the force output by the energy source to the plunger to cause the plunger to act on the stopper to expel the drug from the container, and a trigger member. The system may have pre-injection state where the trigger member inhibits actuation of the mechanical linkage, an injection state where the trigger member permits actuation of the mechanical linkage, and a post-injection state where the plunger is stationarily positioned in an end-of-dose position. The system may be designed to limit the number of components included in various force transmission loops present in the pre-injection, injection, and/or post-injection states.
