Ratchet-based drive systems for more reliable and safer drug delivery are provided. The ratchet-based drive systems restrict angular movement and/or linear movement of components that cause a plunger to expel a liquid drug from a drug container. Movement of the components can be restricted to correspond to a predetermined or desired portion of the liquid drug. In the case that control of the drive system is lost or fails, the maximum amount of drug that could be delivered is limited to a known amount, thereby reducing the likelihood of an overdose.