A medical injector comprising: a body; a reservoir containing first and second mixable components (30,32), a stopper associated with said reservoir, a predetermined extent of advancement of said stopper causing mixing of said first and second mixable components (30,32); a displaceable plunger disposed in said body; a spring disposed to advance said plunger; a first releasable retainer for releasably retaining said plunger in a first state against force of said spring; a first trigger for causing said first releasable retainer to release said plunger; a second releasable retainer for releasably retaining said plunger in a second state against force of said spring; and, a second trigger for causing said second releasable retainer to release said plunger, wherein, said plunger is advanced from said first state to said second state by said spring, said plunger engaging said stopper during said advancement from said first state to said second state so that said stopper traverses at least said predetermined extent of advancement thereby causing mixing of said first and second mixable components (30,32), wherein said second retainer includes a stop block axially shiftable between a blocking state and an open state, wherein, in said blocking state, said stop block interferingly engages a stop member formed on said plunger, and, wherein, in said open state, said stop block permits passage therethrough of said stop member, said shifting of said stop block allowing said plunger to be released from said second state, characterized in that said stop block is axially shiftable under force applied to said second trigger in a direction transverse to the direction of advancement of said plunger. a body; a reservoir containing first and second mixable components (30,32), a stopper associated with said reservoir, a predetermined extent of advancement of said stopper causing mixing of said first and second mixable components (30,32); a displaceable plunger disposed in said body; a spring disposed to advance said plunger; a first releasable retainer for releasably retaining said plunger in a first state against force of said spring; a first trigger for causing said first releasable retainer to release said plunger; a second releasable retainer for releasably retaining said plunger in a second state against force of said spring; and, a second trigger for causing said second releasable retainer to release said plunger, wherein, said plunger is advanced from said first state to said second state by said spring, said plunger engaging said stopper during said advancement from said first state to said second state so that said stopper traverses at least said predetermined extent of advancement thereby causing mixing of said first and second mixable components (30,32), wherein said second retainer includes a stop block axially shiftable between a blocking state and an open state, wherein, in said blocking state, said stop block interferingly engages a stop member formed on said plunger, and, wherein, in said open state, said stop block permits passage therethrough of said stop member, said shifting of said stop block allowing said plunger to be released from said second state, characterized in that said stop block is axially shiftable under force applied to said second trigger in a direction transverse to the direction of advancement of said plunger.