In the United States, EpiPens are only available through a prescription, so retail stores do not offer them to customers over the counter, explains Drugs.com. The EpiPen contains the active ingredient epinephrine, and patients must see a doctor and order the prescription through their pharmacy.
The Epipen, which contains 0.3 milligrams of epinephrine, is designed for adults who weigh more than 66 pounds, while the EpiPen Jr., which contains 0.15 milligrams of epinephrine, is designed for patients between 33 and 66 pounds, according to Drugs.com. For maximum effectiveness, patients must inject an EpiPen as soon as a severe allergic reaction, or anaphylaxis, begins, report the makers of EpiPen. Food allergies, insect stings and medical allergies cause most cases of anaphylaxis.