What are antihistamines?

Antihistamines are medicines that you can take to treat allergies. Allergies and allergic reactions are common. About 1 in 3 Australians have allergies at some point in their lives. Antihistamines work by blocking the effects of a substance called histamine in your body.

What are antihistamines used for?

You can take antihistamines to prevent or relieve your symptoms if you suffer from:

• allergic rhinitis:
  • seasonal allergies (for example, hay fever) cause symptoms in the same month or season each year, and are usually an allergy to grass or tree pollens
  • perennial allergies trigger symptoms all year round, and might involve an allergy to house dust mite, mould or pet hair
• allergic conjunctivitis
• hives (urticaria)
• allergic reactions such as to insect bites or stings, and known food allergies

You shouldn't take antihistamines to treat other itchy rashes such as eczema or psoriasis, which are not caused by histamine.

Some antihistamines can also be used to help with the short-term management of insomnia (sedating antihistamines only).

What types of antihistamines might I be given?

Antihistamines are available in several different forms, for example tablets, nasal (nose) sprays and eye drops.

The best one for you will depend on which symptoms trouble you the most. Talk to your pharmacist or doctor about what treatment is best for you.

There are 2 main types of antihistamines available in Australia that you take by mouth (orally):

• First generation (sedating) antihistamines that can make you feel sleepy include:
  
  
  • dexchlorpheniramine
  • diphenhydramine
  • doxylamine
  • promethazine

Diphenhydramine and doxylamine are used for the short-term treatment of insomnia.

• Second generation (less-sedating) antihistamines that usually do not make you sleepy include:
  
  
  • cetirizine
  • desloratadine
  • fexofenadine
  • loratadine
  • bilastine

In Australia, most antihistamines are available over the counter from pharmacies without a prescription. You should ask your pharmacist for advice if you think you might need to take an antihistamine. They can help you chose an antihistamine that best suits your condition.

Your pharmacist can also check if it's safe for you to take the antihistamine if you are pregnant or take any other medicines.

How do antihistamines work?

Allergies or allergic reactions occur if your immune system mistakes a harmless substance in your environment (for example house dust mites, pet hair or pollen) for a threat and reacts by releasing histamine.

Histamine causes widening of your blood vessels and swelling of your skin. This causes allergic symptoms, such as sneezing, runny nose, watering, itchy eyes or rash. Antihistamines help to prevent or treat your symptoms by blocking the actions of histamine.

Do antihistamines have any side effects or risks?

If you take first generation (sedating) antihistamines, common side effects may include drowsiness and problems with your co-ordination. Most people don't experience side effects from second generation (non-sedating) antihistamines. Other, less common side effects include headache, fatigue, insomnia and rash.

If you've taken antihistamines, you should be careful when operating machinery, including driving a car. Even non-sedating antihistamines can make some people drowsy.

First generation (sedating) antihistamines have anticholinergic properties. This can cause side effects such as dry mouth, blurred vision and constipation. Several studies have been published that suggest anticholinergic medication may be associated with an increased risk of developing dementia, especially in people over the age of 65 years.

Talk to your doctor or pharmacist about the risks before taking first generation (sedating) antihistamines.

You should not use first-generation (sedating) antihistamines for the treatment of cough, cold and flu symptoms in children who are under 6 years. You should not give first-generation (sedating) antihistamines to children under 2 years for any reason. This includes first-generation antihistamine products available over the counter (OTC).

The use of first-generation antihistamines for sleep and behaviour disturbances in children and adolescents can be dangerous and is strongly discouraged.

When should I see my doctor?

You should talk to a pharmacist or doctor before taking an antihistamine if you:

• have kidney or liver disease
• are pregnant or breastfeeding
• have prostate problems

If you think you may have a food allergy or sensitivity, talk to your doctor before taking an antihistamine or excluding a particular food from your diet.

Are there alternatives to antihistamines?

Alternatives are available, but which one your doctor will recommend depends on what is being treated. For example, if you have allergic rhinitis you may benefit from taking steroid nasal spray instead of or in combination with antihistamines. A combination antihistamine and steroid nasal spray is available by prescription.

If you have allergic conjunctivitis you may benefit from taking sodium cromoglycate drops instead of or in combination with antihistamine eye drops.

Your doctor or pharmacist can help you choose an alternative treatment for your symptoms.

Resources and support

For more information and support, try these resources:

• Australasian Society of Clinical Immunology and Allergy information for patients and carers
• Download the AirRater app to track your hay fever symptoms and monitor allergens in the environment.
• Check the pollen count at PollenForkecast, to find the pollen count for your area.
• Asthma Australia explains how hay fever can make asthma worse.
• Allergy and Anaphylaxis Australia information on Urticaria (Hives) and Angioedema