Allergic reactions are severe and potentially life-threatening responses by the body to foreign substances.

Symptoms of this condition include itching of the skin and eyes, swelling of mucous membranes, hives, lowered blood pressure, abdominal pain, vomiting, and vasodilation. Smooth muscles may also be stimulated to contract, restricting breathing. In severe cases, anaphylactic shock may occur, leading to unconsciousness, coma, or death.

The body's symptoms of this condition can be treated through the release of histamine, leukotriene C4, prostaglandin D2, and tryptase. Many allergic reactions share similar characteristics, although they are not mediated by the immune system. Allergic reactions to hormonal drugs such as anabolic-androgenic steroids are extremely rare. However, warnings about this reaction are still often required on labels for many (usually injectable) drugs. Mild skin reactions can be effectively treated with antihistamines. More severe conditions may require IV epinephrine and other supportive treatments. Given the potential for rapid development, immediate medical attention is necessary if an allergic reaction occurs.