An injection device 110 has a mechanism for restraining and triggering a drive spring 130 to operate a syringe 114 guided by a syringe carrier 127. The mechanism also prevents forward movement of the syringe carrier before triggering. A trigger switch 102 mounted ON the housing 112 between the drive spring and syringe carrier pivots about a point 102a intermediate its ends the proximal end of the trigger has a locking surface 102b which engages a drive sleeve 131 and prevents extension of the spring the distal end carries an annular member 170 which engages with slots ON the syringe carrier and in a distal portion 112of a the housing 112 to restrain the syringe carrier. Upon triggering and drive is transmitted to the syringe via two rods 132 and 134 connected by a piston so as to cause a time delay.
