What conditions are antihistamines used to treat?
Antihistamines are commonly used:
- To relieve the symptoms associated with hay fever (seasonal allergic rhinitis). These can include:
- To reduce the severity of the rash and itching associated with nettle-type rashes such as hives (urticaria) and generalised itching (pruritus).
- To help with the rash and itching following insect stings or bites.
- To prevent motion sickness and other causes of feeling sick (nausea).
- Occasionally to treat severe morning sickness in pregnancy.
- In the care of the terminally ill, for their sedating and antisickness effects.
- In the emergency treatment of severe allergic reactions.
How do antihistamines work?
Histamine is a chemical naturally produced by various cells in your body. It has a variety of different functions. Large amounts of histamine are made in cells called mast cells, in places where the body comes into contact with the outside environment. For example, in the nose, throat, lungs and skin. Here, mast cells and histamine form part of your immune defence system. (Whereas, in the stomach, histamine made by cells that line the stomach helps to produce acid for food digestion.)
Your immune system cells monitor your blood and mucosae for anything (for example, germs such as bacteria or viruses) that is not made by your body. (Mucosae are membranes lining body cavities such as your mouth, nose and digestive tract.) If your skin is damaged or your immune system detects a foreign substance, histamine is released from mast cells. The histamine binds to special sites (receptors) on other cells, called H1 receptors. This sets off a chain reaction which causes blood vessels in the area to become slightly leaky. Specialised cells and chemicals, which defend your body, can now get access to the area. While this is a helpful response, it also causes redness, swelling and itching.
Allergic reactions such as hay fever (seasonal allergic rhinitis) are caused by an oversensitivity or over-reaction of the immune system to a particular allergen. An allergen is a substance that is foreign to the body and which can cause an allergic reaction in certain people. For example, pollen, dander, mould, some germs. In most people, the immune reaction to these foreign substances is normal and appropriate. But in allergic people, it is excessive. For example, in people with hay fever, contact with pollen in the nose, throat and eyes triggers the mast cells there to release much more histamine than normal. This excessive release of histamine produces the associated symptoms of itching, swelling, runny eyes, etc.
Antihistamines work by physically blocking the H1 receptors, stopping histamine from reaching its target. This decreases your body's reaction to allergens and therefore helps to reduce the troublesome symptoms associated with allergy.
Antihistamines are also used in the treatment of feeling sick (nausea) and being sick (vomiting). However, the exact way that they ease these symptoms is not fully understood. The brain has several key areas which control vomiting. It is thought that antihistamines block H1 receptors in the area of the brain which creates nausea in response to chemicals in the body.
Some antihistamines may also have what is known as an antimuscarinic effect. This means that the medicine can also block another type of receptor found on the surface of certain cells. If these receptors are affected, you may experience some of the side-effects associated with antihistamines. For example, dry mouth, blurred vision and retention of urine. These effects are mainly caused by the older first-generation antihistamines which are described below.
Note: antihistamines should not be confused with H2 blockers which reduce the production of stomach acid. While both types of medicine block the actions of histamine, they work on different receptors in different systems of the body.
Further reading and references
British National Formulary; NICE Evidence Services (UK access only)
Allergic Rhinitis; NICE CKS, October 2015 (UK access only)
Urticaria; NICE CKS, May 2016 (UK access only)
Guidelines for the management of allergic and non-allergic rhinitis; British Society for Allergy and Clinical Immunology (January 2008)
Primary Care Rhinitis algorithm; British Society for Allergy and Clinical Immunology (BSACI)
Over-the-counter cough and cold medicines for children; Medicines and Healthcare products Regulatory Agency (MHRA), 2009
Hi!I'm lost and confused....I am 50, and in the last year I have developed 5 new food allergies--soy, stone fruits, onions, coconut and casein. Does anyone know why such an avalanche of allergies...maddysmom2015
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