What are antihistamines?

Antihistamines are medicines which can be used to relieve the symptoms of allergies e.g., hay fever, hives, conjunctivitis, skin rashes and reactions to insect bites or stings.

However, they can also be used to prevent motion sickness, treat nausea or vomiting and as a short-term treatment for insomnia.

Mechanism of action

Antihistamines work by blocking the effects of a substance called histamine in your body by blocking it from binding to its receptors. Normally, histamine is released when the body senses something harmful, like an infection, causing blood vessels to expand and the skin to swell to protect the body. However, in people with allergies, the body mistakenly identifies harmless substances like pollen or animal hair as threats, triggering histamine release and allergic reactions such as itchy eyes, runny nose, sneezing, and skin rashes.

By taking antihistamines before encountering the allergen or after, they can prevent or lessen the severity of these symptoms.

What types of antihistamine are there?

There are many types of anti-histamine. They can be divided into 2 categories - drowsy (can make you feel sleepy) e.g., chlorphenamine and non-drowsy e.g., cetirizine, fexofenadine and loratadine.

They also come in a variety of medicinal forms such as tablets, solution, eye drops and nasal sprays.

How to choose the best antihistamine

In general, there isn’t much evidence to show that one antihistamine is more favourable than the other. Several antihistamines may have to be tried in order to find one that works well for a patient.

Non-drowsy antihistamines may be preferred as they’re less likely to make a patient feel sleepy. Although, a drowsy antihistamine may be more suitable if allergy symptoms are affecting sleep.

Usually, loratadine is the recommended antihistamine during pregnancy. Both loratadine and cetirizine are suitable during breastfeeding.

Side effects

Generally these are the side effects of antihistamines; however, the patient information leaflet for each different medicine will outline its side effects.

Drowsy antihistamines:

• Sleepiness and reduced co-ordination, reaction time and judgement – do not drive or use machinery after taking these antihistamines 😴 

• Dry mouth 👄 

• Blurred vision 👀 

• Difficulty peeing 🚽 

Non-drowsy antihistamines:

• Headaches 🤕 

• Dry mouth 👄 

• Feeling sick 🤢 

• Sleepiness – this is less common with non-drowsy antihistamines but is still possible 😴 

Taking antihistamines with food, alcohol and other medicines

Patients should be advised to try to not drink alcohol while taking an antihistamine, especially a drowsy antihistamine as this can increase the chances of it making you sleepy. 🍷 

Food and other drinks don’t affect most antihistamines.

Fexofenadine + indigestion remedies = Do not take indigestion remedies 2 hours before or after taking this medicine.

Avoid taking cough and cold remedies also containing an antihistamine at the same time.

Tricyclic antidepressants such as amitriptyline may also interact with antihistamines such as chlorphenamine. Both of these medicines can cause antimuscarinic effects and sedation.

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