A drug delivery device (100), including a body (104, 116) having a reservoir (160) disposed therein for containing a medicament, , the reservoir including a flexible wall (164)； a plunger (144, 508, 564) movable within the body (104, 116) for causing the medicament to be expelled from the reservoir (160), the plunger (144, 508, 564) having a contact surface that is not affixed to said flexible reservoir wall (164)； a spring (140) biasing the plunger (144, 508, 564) toward the reservoir (160)； and means (200, 204, 216, 218, 504, 516, 520, 524, 564, 568, 572, 582, 584) for selectively maintaining the plunger (144, 508, 564) in a pre-activated position with respect to the reservoir (160) and, upon releasing the plunger (144, 508, 564) from the pre-activated position, for guiding the plunger (144, 508, 564) to move under the force of the spring (140) such that the contact surface of the plunger (144, 508, 564) contacts the flexible reservoir wall (164) to pressurize the reservoir (160) for delivery of the medicament to a patient.
