Antihistamines are drugs which combat the effects of histamine. Histamine is a chemical released by certain cells of the body (mast cells):

• during an allergic reaction;
• when reacting to mechanical trauma of the skin;
• in response to stings and insect bites.

The antihistamines are helpful for treating:

• Hayfever (seasonal allergic rhinitis)
• Other nasal allergies
• Itch and itchy rashes including urticaria or hives.
• Insect bites and stings
• Allergy, including severe forms

Sometimes we make use of their side effects (only specific antihistamines):

• To reduce nausea and sickness, including motion sickness
• To help sleep
• To dry secretions

Antihistamines specifically overcome (antagonize) the effects of histamines. They can help to treat conditions where histamine is the chemical involved in producing the symptoms.

Each antihistamine has its own side effects, and you should read the leaflet that comes with them, as well as discussing your treatment with your doctor or pharmacist.

Because of their action, their are certain side effects that often occur, but clever changes in the structure of the chemicals has enabled some antihistamines to have less of the unwanted effects. Your doctor or pharmacist will be able to advise on the one most suitable for you.

The most common adverse effect used to be drowsiness, and most of the older antihistamines do tend to make people drowsy. As a result they also react with alcohol and other drugs which have sedative effects. Sometimes this can be a positive benefit eg reducing itch, and giving a better night's sleep when the patient is suffering from an intense irritation; also, occasionally, in helping a child to sleep.

A new group of antihistamine drugs was developed which are not drowsy-making. Some of the first generation of these were found to cause heart rhythm problems (arrhythmias) which could be serious, but new, safer alternatives are now used.

Read the leaflet with the medication and consult the pharmacist or your doctor.

The main forms used are tablet, capsule and liquid. In severe allergic reaction, injection may be used.

There are some antihistamine creams aimed at stings etc. They are only slightly effective, and may lead to the body becoming allergic to that antihistamine, which could cause severe reactions if the same antihistamine is given later by mouth or injection.