Antihistamines are medicines often used to relieve symptoms of allergies, such as hay fever, hives, conjunctivitis and reactions to insect bites or stings.
Most antihistamines can be bought from pharmacies and shops, but some are only available on prescription.
“Antihistamines block the effects of a substance called histamine in your body,” said the NHS.
The health site added: “Histamine is normally released when your body detects something harmful, such as an infection.
“It causes blood vessels to expand and the skin to swell, which helps protect the body.”