Antihistamines are a type of medicine often used to treat a number of allergic health conditions.
allergic rhinitis – inflammation of the nose caused by an allergic reaction to substances such as dust mites
allergic skin conditions, such as eczema or urticaria (hives)
allergic conjunctivitis – inflammation of the eyes
Antihistamines are available in tablet or capsule form (oral antihistamines), creams, lotions and gels (topical antihistamines) and as a nasal spray.
Many antihistamines are available over the counter at a pharmacy, although some require a prescription.
How antihistamines work
Antihistamines work by altering the way that cells are affected by a substance called histamine. Histamine is a chemical that the immune system uses to help protect the body’s cells against infection.
Usually histamine is a useful substance, but if you’re having an allergic reaction it’s sometimes necessary to block its effects. Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, for a threat.
Types of antihistamine and their effects
Antihistamine medicines are classified in three groups. These are:
first-generation antihistamines, which cause symptoms of drowsiness in most people and include diphenhydramine and chlorphenamine
second-generation antihistamines, which do not usually cause symptoms of drowsiness and include loratadine and cetirizine
third-generation antihistamines, which cause fewer serious side effects than second-generation antihistamines and include levocetirizine and fexofenadine
Second- or third-generation antihistamines are usually recommended. Do not underestimate the levels of drowsiness caused by first-generation antihistamines – their effects can continue into the next day if you only take them at night.
An exception to these recommendations is sometimes made if the drowsiness caused by first-generation antihistamines can be beneficial, such as cases where people have problems sleeping because of itchy skin.
Even though most antihistamines are available without a prescription, you shouldn’t assume that they’re safe for everyone to take.
For example, antihistamines may have dangerous and unpredictable effects if taken by people with certain conditions or if combined with certain other substances.
It’s also important to only take antihistamines as directed. Overdoses are possible and overuse can lead to you becoming reliant on the sedating effects.
Before taking antihistamines, always read the patient information leaflet that comes with the medicine to check the safety information.