As the name implies, antihistamines counteract the effects of histamine – one of the principal chemicals released in an allergic reaction. The following article is intended for general information, but always follow the advice of your doctor or another healthcare professional and always read the label of any medicine prescribed.
Antihistamines are best known for their use in controlling allergic conditions such as hay fever, but they can also be effective in tackling the irritating symptoms of mild food allergies. Many are available over the counter in a number of forms and your doctor or pharmacist should advise on which may suit you.
Chlorphenamine (Piriton) is an ‘old school’ antihistamine (also known as a first generation antihistamine), which can make you sleepy. You should not drive or operate machinery if this medication makes you feel drowsy or dizzy.
Antihistamines come in tablet or liquid form. Tablets have the advantage of being easy to carry, but syrups can be taken when you don’t have water to hand. Syrups, usually sweetened with sucrose, tend to be favoured for children, to whom a precise dose can be given. Where an antihistamine contains sucrose, it is recommended that the teeth should be cleaned after each dose. Some chemists now offer ‘fast-melt’ antihistamines which can be taken without water and dissolve easily in the mouth.
Chlorphenamine and Cetirizine are fast acting antihistamines and take 15-20 minutes to work. Some others take much longer.
For anyone at risk of a severe allergic reaction (known as anaphylaxis), an adrenaline auto-injector (AAI) will be prescribed as well as antihistamines. The antihistamines are intended to treat mild symptoms, but adrenaline should always be used promptly when anaphylaxis is suspected. See our separate fact sheets on anaphylaxis and adrenaline.
As with all medicines, antihistamines may not suit everybody. Let your pharmacist know if you are taking other medicines or have chronic or short-term medical conditions. All antihistamine tablets sold in the UK contain lactose. People who are severely allergic to milk or are lactose intolerant may find this can cause problems. Although pharmaceutical-grade lactose should be free from cows’ milk protein, some people still experience symptoms. Anyone affected in this way may wish to use the syrup formulations, which do not contain lactose. Check this with your pharmacist.
Antihistamine syrups may be inadvisable for people with diabetes or a sugar intolerance. Some antihistamines should not be taken by young children, or those with decreased liver or kidney function. Pregnant or nursing women should avoid certain antihistamines and are advised to speak to a doctor for guidance and advice. Other groups who should be vigilant in checking the safety of their medication include epileptics, the elderly, glaucoma sufferers, asthmatics and those with cardiovascular disease or high blood pressure.
Following research from America (Gray, Anderson et al. 2015) elderly patients taking antihistamines long term should consult their doctor to ensure they are not at increased risk of developing dementia.
Parents of young people who are studying should be aware that some antihistamines that cause drowsiness have been shown to adversely affect their exam marks. However, it should also be pointed out that hay fever can also affect exam performance (Walker, Khan-Wasti et al. 2007).
The text of this article has been peer reviewed by Sue Clarke, Specialist Allergy Health Visitor.
Disclosure: In January 2019 Sue carried out consultancy work for Bausch and Lomb, the company that markets the Emerade adrenaline auto-injector.
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Publication date: Dec 2019
Review date: Dec 2022