An actuation mechanism 10 for an automatic injector 100 includes a first actuation spring 102, a first actuation pill 104, a second actuation spring 106, and a second actuation pill 108. In an initial configuration the first actuation spring 102 resides in a compressed, energized state substantially within an upper portion of first actuation pill 104 and the second actuation spring 106 resides in a compressed, energized state between the first actuation pill 104 and the second actuation pill 108. An automatic injector 100 includes a housing 14, 16, an activation mechanism 12, an actuation mechanism 10, and a syringe cartridge 20 having a plunger 200 and a needle assembly 40. A retractable syringe may be utilized as a syringe cartridge 20, such as a prefilled retractable syringe with integrated safety features which retract the needle 420 after use. The automatic injector 100 may be utilized to inject the needle 420, deliver a drug treatment, and activate retraction of the needle 420. Methods of assembly and operation are also provided.本發明揭露一種用於一自動注入器(100)之致動機構(10)，其包括一第一致動彈簧(102)、一第一致動終端片(104)、一第二致動彈簧(106)、及一第二致動終端片(108)。於一初始組態中，該第一致動彈簧(102)係以壓縮的激勵態實質上駐在第一致動終端片(104)之一上部內部，而該第二致動彈簧(106)係以壓縮的激勵態駐在該第一致動終端片(104)與該第二致動終端片(108)間。自動注入器(100)包括一殼體(14、16)、一啟動機構(12)、一致動機構(10)、及具有一柱塞(200)及一針頭總成(40)的一注射器卡匣(20)。一伸縮型注射器可用作為一注射器卡匣(20)，諸如預充式伸縮型注射器附有整合型安全結構，該安全結構可在使用後回縮針頭(420)。該自動注入器(100)可利用來注射針頭(420)，遞送藥物處理，及啟動針頭(420)的回縮。也提供組裝與操作方法。
