Antihistamines are a type of medicineoften used totreat a number of allergic health conditions.

Although antihistamines can't cure these conditions,they often provide relief from symptoms. For example, antihistamines may be used to treat:

Antihistamines also have a number of other uses, such as treating stomach ulcers , insomnia (problems falling asleep) and motion sickness .

Antihistamines are availableas

  • tablet or capsules (oral antihistamines)
  • creams, lotions and gels (topical antihistamines)
  • 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 cells are affected by a substance called histamine. Histamine is a chemical 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.

These are:

  • first-generation antihistamines which cause drowsiness in most people and include diphenhydramine and chlorphenamine
  • second- or third-generation antihistamines which are less likely to cause drowsiness and include loratadine and cetirizine

Second- or third-generation antihistamines are usually recommended. Don't underestimate the levels of drowsiness caused by first-generation antihistamines their effects can continue into the next day if you take them at night.

An exception to this is sometimes made if the drowsiness caused by first-generation antihistamines can be beneficial, for example in cases where itchy skin may be causing sleep problems.

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.

and interactions of antihistamines .

Medically Reviewed by a doctor on 4 Jan 2017