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Antihistamine drugs

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 4 minutes read

Antihistamines are a group of medications that prevent the level of histamine from rising in the body and reduce the symptoms this can cause. Antihistamines can be used to treat a range of conditions from allergic reactions, motion sickness and insomnia to insect bites, allergic conjunctivitis and nausea. However, the term "antihistamines" most commonly refers to medication used to treat hay fever.

Some of the antihistamines that are available to buy over-the-counter in the US include: chlorpheniramine, Claritin (the active drug is loratadine), Xyzal (the active drug is levocetirizine), Zyrtec (the active drug is cetirizine), Allegra (the active drug is fexofenadine) and Benadryl (the active drug is diphenhydramine). These are generally licensed for the treatment of hay fever, allergic reactions to pets, dust mites and food, insect bites, chickenpox rash and prickly heat rash. They’re available as creams, tablets and oral liquid.

Doctor’s advice

Who should take antihistamines?

If you’re suffering from hay fever or allergic-type symptoms such as an itchy nose, itchy rash or bite on the skin or redness and minor swelling of the skin, then an antihistamine may help relieve these symptoms.

Minor allergic reactions to products (washing detergent, creams, perfume), food or plant reactions (stinging nettles) can be safely treated with over-the-counter antihistamines and should get better in hours to days but make sure to avoid whatever caused the reaction in the future.

If you find that you’re getting regular allergic reactions and don’t know the reason why, you should discuss with your doctor, and you may benefit from allergy testing.

How do they work?

Histamine is a naturally occurring chemical in the body that is activated and released from your cells when your immune system is triggered by a foreign substance (such as pollen). Histamine sets off a chain reaction, allowing your immune system to detect and remove the thing that caused it.

While this is a useful and important response, the release of histamine can also cause redness, swelling and itchiness that can be bothersome. People that suffer from hay fever or other allergies release a higher-than-normal amount of histamine from their immune system.

Antihistamines help to prevent and reduce the effect of histamine in the body, and therefore are effective at relieving allergic symptoms.

Antihistamines can be classified as drowsy or non-drowsy. Chlorpheniramine can make you drowsy so care should be exercised when driving or operating heavy machinery, and alcohol should be avoided. Some antihistamines that cause drowsiness are also found in over-the-counter sleeping tablets such as diphenhydramine.

All other over-the-counter antihistamines are classed as non-drowsy: cetirizine, levocetirizine, fexofenadine, and loratadine. These are less likely to cause drowsiness, but it is still a possible side effect.

How do I take antihistamines?

Different medications have different doses and schedules for how often you can take them. Check with your pharmacist or read the box carefully if you are not sure.

Most antihistamines are safe to take for long periods of time - for example throughout the hay fever season - but chlorpheniramine should not be used for longer than two weeks at a time without consulting a doctor. Antihistamine creams are also available to buy and may be more suitable for treating localized reactions such as insect bites or stings.

Talk to your doctor if you are pregnant or breastfeeding before taking antihistamines. Do not take any antihistamines three days before taking an allergy skin test as this can affect your results.

Like all medications, don’t take them if you have previously had an allergic reaction to the medication, if you have severe kidney or liver problems, or if you have other medications or medical conditions where you would normally discuss with your doctor or pharmacist before starting something new. Avoid if you have hypertension, cardiovascular disease, urinary retention, or glaucoma.

Are there any side effects?

Common side effects include drowsiness. These are more common with chlorpheniramine and diphenhydramine, but may also be experienced with non-drowsy antihistamines, too. Other side effects include dizziness and headache.

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Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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