In these conditions they work by preventing the actions of histamine, which is a substance produced by the body as part of its natural defences.It's stored in cells called mast cells, in almost all tissues of the body, and is released when the body reacts to a foreign substance (known as an allergen).
Medicines that are not totally essential should ideally be avoided during pregnancy, and pregnant women should not take antihistamines unless prescribed by a doctor.
However, if itching is still a problem your doctor may prescribe you an antihistamine.
Antihistamines are most commonly used to control the symptoms of allergies such as hay fever.
The released histamine binds to its receptors (H-1 receptors) causing a chain reaction that includes an increase in blood flow to the area, and the release of other chemicals that add to the allergic response. Antihistamines work by blocking histamine receptors, therefore reducing the reactions that cause itching.
However, histamine is only one of many substances in the body that cause itching, and these medicines are mainly of value in eczema because they cause sedation.