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Antihistamines

Antihistamines work by stopping histamine affecting your body's cells in the usual way.

Antihistamines work by stopping histamine affecting your body's cells in the usual way. They target special molecules called receptors, which are found in your cells.

Histamine

Histamine is a chemical the immune system uses to help protect the body's cells against infection. The immune system is the body's natural defence against illness and infection.

If the immune system detects a harmful foreign object, such as bacteria or a virus, it will release histamine into nearby cells. The histamine causes small blood vessels to expand and the surrounding skin to swell. This is known as inflammation.

Histamine is usually 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.

Receptors

Receptors are molecules found in the cell walls. They react when they come into contact with certain substances.

Antihistamines work by blocking the receptors in each cell, so histamine can't activate the receptors and affect the cell.

Histamine receptors

Histamine receptors cause inflammation and stimulate the production of stomach acid. They're also thought to help stimulate chemicals that transmit information around the brain and may help regulate the immune system.

The majority of antihistamines are designed to block the receptors causing inflammation. Antistamines used to treat stomach ulcers are designed to block the receptors producing stomach acid.

Research is underway to produce an antihistamine which can be used to treat mental health conditions and neurological conditions, such as Alzheimer’s disease and autoimmune conditions, such as rheumatoid arthritis.


Content Supplied by NHS Choices

Antihistamines

Most people are able to take antihistamines. However, antihistamines are not recommended in certain circumstances.

Most people are able to take antihistamines. However, antihistamines are not recommended in certain circumstances.

These are explained below.

Health conditions

Certain conditions can be made worse by taking antihistamines, or they can cause the antihistamines to react unpredictably. These include:

Before taking antihistamines, seek advice from your GP or pharmacist if you have a health condition.

Pregnancy

As a general rule, avoid taking any medication during pregnancy unless there's a clear clinical need. Always check with your GP, pharmacist or midwife first.

If you're pregnant and feel you need antihistamines, nasal sprays, nose drops or eye drops will at first be recommended. If these don't work, an oral antihistamine, usually loratadine or cetirizine is likely to be recommended.

Chlorphenamine is also considered safe to use during pregnancy, but should be avoided close to labour and childbirth as it can cause problems in the baby, such as irritability or tremor (shaking).

Read more information about taking hay fever medication during pregnancy.

Breastfeeding

It may be possible for you to take some hay fever medicines while you're breastfeeding without risk to your baby. 

However, you should always get advice from your pharmacist, GP or health visitor first.

Read more information about taking hay fever medication while breastfeeding.

Children

Some antihistamines, such as alimemazine and promethazine, aren't suitable for children under two years old. You should seek advice from your GP if your child is under two years old and you think they require treatment with antihistamines.

Some antihistamines are not recommended for children with certain conditions. For example, the National Institute for Health and Care Excellence (NICE) recommends oral antihistamines aren't used routinely to treat children with atopic eczema.

Before giving your child any form of medication, read the patient information leaflet for advice about whether the medication is suitable for them.


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