An epinephrine autoinjector is a medical device for injecting a measured dose or doses of epinephrine (adrenaline), by means of autoinjector technology; the EpiPen is most often used for the treatment of anaphylaxis. The EpiPen is derived from the Mark I NAAK ComboPen, which was developed for the U.S. military for treating exposure to nerve agents in the course of chemical warfare. Other trademark names for the autoinjector device include, Emerade, Twinject, Adrenaclick, Anapen, Jext, Allerject, and Auvi-Q.
The devices contain a spring-loaded needle that exits the tip of the device (in some cases through a sterile membrane) and penetrates the recipient's skin, to deliver the medication via subcutaneous or intramuscular injection.
Epinephrine autoinjectors contain a pre-determined dose of epinephrine, usually between 300 μg and 500 μg of active ingredient at a concentration of 1:1000. They typically contain more medication than the amount needed for a single dose, but any extra amount is not intended for use and is inaccessible without destroying the device. Manufacturers have also made pediatric dosed versions available at 150 μg of active ingredient. There was a version that contained two individual doses (in case a repeated application is required) previously sold under the trade name Twinject. The company that produced Twinject autoinjectors, Shionogi, announced that it was discontinuing Twinject effective March 30, 2012.
On August 13, 2012, the U.S. Food and Drug Administration (FDA) approved a new version of epinephrine auto-injector made by Intelliject and Sanofi called the Auvi-Q. It is rectangular in shape, 3.5 inches by 2 inches by 0.5 inch. It has a soundchip in it to give audio cues to a patient or caregiver to aid in the proper use of the device.
In most countries, epinephrine is a prescription drug, and therefore obtaining the device requires a prescription from a doctor. However, in some jurisdictions, epinephrine autoinjectors are an over-the-counter drug and may be purchased from a pharmacy counter.[where? — see talk page]
After activation, the patient holds the device in place for between 5 and 10 seconds as the epinephrine is delivered. This gives the drug enough time to be absorbed by the body's muscles and diffused into the bloodstream.
Auto-injectors are sometimes used unnecessarily. Injection into a vein (intravenous injection) can be fatal. It can cause ventricular tachycardia, in which the heart beats uncontrollably and is not able to pump blood adequately. It can also restrict blood flow to the area of the injection site, and damage the extremities. After administering the device, patients are advised to seek immediate medical attention.
An emergency technique (not manufacturer-approved) to obtain additional epinephrine from a used autoinjector has been published in the medical literature. 
Units that have exceeded their expiration date can still be used in an emergency if an unexpired unit is unavailable.
None of these devices prevent future episodes of anaphylaxis. Patients who experience severe or life-threatening reactions may be treated further via allergen immunotherapy for long-term protection. A series of allergy injections composed of increasing concentrations of naturally occurring substances (such as venom) provide excellent and usually lifelong protections against adverse effects of future insect stings. The initial injections have a very low concentration, usually 1 ppb or less.
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