Antihistamines are useful for mild allergic symptoms such as itching, hives, sneezing, and a runny nose, because they block histamine, one of the chemicals released during an allergic reaction. They are a sensible part of managing hay fever and minor skin reactions.
However, antihistamines are not a treatment for anaphylaxis. They work too slowly and do not reverse the dangerous effects on breathing and blood pressure. Relying on an antihistamine during a severe reaction can delay the treatment that actually saves lives. Adrenaline (epinephrine), given by auto-injector into the outer thigh, is the only first-line treatment for anaphylaxis and should be used without hesitation when severe symptoms appear.
If someone has signs of a severe reaction, such as difficulty breathing, throat or tongue swelling, widespread hives with faintness, or collapse, give adrenaline if available and call your local emergency number immediately. An antihistamine can be given afterwards for any remaining itch, but never instead of adrenaline. A doctor can advise on what to carry and when to use each medicine.