A jet injector that includes a prefilled syringe. The syringe includes a fluid chamber that contains a medicament. The syringe also has an injection-assisting needle, and a plunger is movable within the fluid chamber. A housing is configured for allowing insertion of the needle to a penetration depth. The housing includes a retractable guard and an interference component, e.g., a lock ring, adjacent to the retractable guard that interferes with the movement of the retractable guard. An energy source is configured for biasing the plunger to produce an injecting pressure to jet inject the medicament from the fluid chamber through the needle to an injection site.