An autoinjector is a medical device designed to deliver a single dose of a particular (typically life-saving) drug. Most are spring-loaded syringes. By design, autoinjectors are easy to use and intended for use by laypersons. Depending on the drug loaded, the autoinjector is pressed against different places in the body; typically, the thigh or the buttocks are the safest if you must act but do not know.
One brand of autoinjector for epinephrine is an Epi-pen, which is often prescribed to persons who have survived anaphylaxis.
In the United States Army, autoinjectors loaded with atropine are used for first aid against chemical weapons, particularly nerve agents. In the Gulf War, accidental and unnecessary use of atropine autoinjectors proved to be a major medical problem.
Combat lifesavers and some other soldiers are issued autoinjectors loaded with morphine, or Syrettes, to administer to severely injured persons for pain control. Special operations soldiers have been known to inject multiple Syrettes immediately prior to capture in order to avoid immediate interrogation at the severe risk of an overdose, as every hour of delay increases the chance of rescue before interrogation.