What are antihistamines?

Antihistamines are medicines that relieve the symptoms of allergies. When you have an allergic reaction, your body releases a chemical called histamine, which causes symptoms like sneezing, itching and a runny nose. Antihistamines work by blocking histamine, calming these symptoms down.

What antihistamines are used for

Antihistamines are commonly used for:

  • hay fever and other allergies
  • hives (urticaria) and itchy skin
  • reactions to insect bites and stings
  • allergic conjunctivitis (itchy, watery eyes)

Drowsy and non-drowsy types

There are two broad groups. Older antihistamines can make you feel sleepy, which is why some are also used to help with sleep. Newer ones are usually “non-drowsy” and are generally preferred for daytime use, such as managing hay fever while at work. A pharmacist can help you choose.

What are the side effects of antihistamines?

Side effects depend on the type. Drowsy (older) antihistamines can cause:

  • sleepiness, and slower reactions, co-ordination and judgement, so do not drive or use machinery after taking them
  • a dry mouth
  • blurred vision
  • difficulty peeing

Non-drowsy antihistamines can cause:

  • headaches
  • a dry mouth
  • feeling sick
  • drowsiness, which is less common but still possible

The leaflet in the pack lists every side effect and when to get help. You can report a suspected side effect through the MHRA Yellow Card scheme.

Using them safely

Antihistamines are available in tablets, liquids, nasal sprays and eye drops. To use them well:

  • follow the instructions on the packet or your prescription
  • if using a drowsy type, do not drive or operate machinery until you know how it affects you
  • tell a pharmacist about other medicines you take, as some can interact

Always read the patient information leaflet.

What antihistamines cannot treat

Antihistamines help with everyday allergy symptoms, but they are not a treatment for a severe allergic reaction (anaphylaxis). If someone has swelling of the face, lips or throat, difficulty breathing, or feels faint, this is an emergency — call 999 immediately.

Common questions about antihistamines

What are antihistamines used for?
They are mainly used for allergies — hay fever, allergic reactions, hives and itchy skin, and reactions to insect bites and stings. Some are also used for travel sickness or to help with sleep, though that is a separate use.
What is the difference between drowsy and non-drowsy antihistamines?
Older "first-generation" antihistamines can make you sleepy, while newer "second-generation" ones usually do not. For daytime allergy relief, a non-drowsy type is often preferred. A pharmacist can advise which suits your needs.
Can I take antihistamines every day?
Many people take a non-drowsy antihistamine daily during allergy season, and this is generally fine. If you find you need them long term or they are not controlling your symptoms, speak to a pharmacist or GP.
Do antihistamines treat a severe allergic reaction?
No. Antihistamines help mild to moderate allergy symptoms but are not a treatment for anaphylaxis — a severe, life-threatening reaction. That needs emergency treatment (adrenaline) and a 999 call.

Sources

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