Disposable auto-injector for prefilled syringes

An auto-injector for a medicament prefilled syringe (4), having a head at one end and fitted with a needle at the other end and including a syringe piston (23), said auto-injector incorporating a first device D1 which makes automatic the needle penetration into a user's body and controls a second device D2 which performs the medicament injection, said first device D1 comprising:

• a cylindrical body (1) formed for receiving the syringe (4), said cylindrical body being provided at an upper end with a collar forming a seat suitable to bear the head of the syringe,
• a slider (5) concentric to said body and adapted to slide on a cylindrical surface of said body,
• a first trigger means (9) arranged such that a reciprocal movement of said body and said slider is prevented, thus maintaining the slider in a position to cover the entire needle (10) of the syringe, the first trigger means being releasable when a predetermined force is exerted on said slider, thus allowing reciprocal movement of said body and said slider, said slider reaching a position whereat said needle is uncovered and penetrates into the user's body,
• said second device D2 comprising:
• a piston (21), resting against the syringe piston (23) and urged by a compressed spring (22),
• a second trigger means (19), retaining said piston (21), wherein one of said body and said slider is formed with a bevelled projection and the other of said body and said slider is formed with a shoulder (11), said first trigger means (9) being partly placed between said projection and said shoulder, said slider (5) of said first device D1 being fitted with an appendix disposed for releasing said second trigger-means (19) of said second device (D2) when said reciprocal movement of said body (1) and said slider (5) is completed with the result of having the medicament injection beginning only after the complete needle penetration thereby avoiding any medicament delivery at a non-desired depth or site.