Antihistamines are a type of medicine often used to treat 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:

• hay fever 
• 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
• allergic reactions caused by insect bites or stings
• mild or moderate allergic reactions caused by food allergies – more severe allergic reactions (anaphylaxis) usually require emergency treatment with adrenaline

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

Antihistamines are available as

• 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.

Read more about how antihistamines work.

Types of antihistamine and their effects

Antihistamine medicines are classified in three groups. 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.

Read more about the side effects of antihistamines.

Safety

Even though most antihistamines are available without a prescription, you shouldn't assume they're safe for everyone to take.

Antihistamines may have dangerous and unpredictable effects if taken by people with certain conditions or if combined with certain other substances, such as alcohol or certain antidepressants.

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.

Read more about who can use antihistamines and interactions of antihistamines.