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First-generation antihistamines

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 3 minutes read

Antihistamines are commonly used to help treat allergy symptoms but can also treat various conditions such as insect bites, motion sickness, or even to help with sleep. They reduce sneezing, a runny nose, skin rashes, or itching but are usually less effective for nasal congestion. Antihistamines are widely available over-the-counter in many pharmacies and shops, but some are only available on prescription.

There are two types of antihistamines – first-generation (drowsy) and second-generation (non-drowsy). First-generation antihistamines are the first kind that were developed. They can make you sleepy and are useful if your allergy symptoms keep you up at night. They also help with travel or motion sickness. Common first-generation antihistamines include chlorpheniramine and diphenhydramine (Benadryl).

Doctor’s advice

Who is it for?

Antihistamines are useful to ease your allergy symptoms and are beneficial if you suffer from allergic conditions like hives, drug allergies, itchy skin conditions, hay fever, and more. They can also help ease coughs and colds, nausea and motion sickness, vertigo, and insect bites.

The most common side effect of first-generation antihistamines is drowsiness which can be especially useful if your symptoms affect your sleep. Dimenhydrinate (Dramamine) can be purchased over the counter for travel sickness.

How do they work?

Antihistamines work by blocking the actions of a chemical called histamine in the body. The body’s immune system makes histamine to attack allergens that you are exposed to. Too much histamine can cause irritation and inflammation in many organs, such as the nose, throat, lungs, and skin, resulting in the traditional “allergy symptoms” of itchiness, swelling, and redness.

First-generation antihistamines differ from their newer counterparts due to their chemical structure, allowing them to cross the blood-brain barrier and block the histamine receptors in the brain. This results in a sedating side effect, unlike second-generation antihistamines, which selectively target peripheral histamine receptors. First-generation antihistamines also target the part of the brain that controls nausea and vomiting, and thus can also be used to prevent motion sickness.

Should anyone avoid taking it?

First-generation antihistamines may cause drowsiness, dizziness, and blurred vision. Do not drive or use heavy machinery after taking these medicines. Concurrent use of alcohol should also be avoided, as this enhances the drowsy effects. Like any other medicine, do not take these if you are hypersensitive to antihistamines or any of the other ingredients.

Medical advice from a doctor or pharmacist should be sought before purchasing antihistamines for children or the elderly, those who are pregnant or breastfeeding, people suffering from serious medical conditions (epilepsy, high blood pressure, glaucoma, heart disease, breathing problems, enlarged prostate, kidney/liver disease, etc.), and people that use other prescription medications.

Are there any side effects?

Some common side effects include:

  • Drowsiness, dizziness, and reduced coordination and reaction speed
  • Tiredness
  • Headaches
  • Blurred vision
  • Dry mouth
  • Difficulty passing urine

The leaflet with your medicine will give a full list of possible side effects and advice about when to get medical help, depending on what product you have purchased.

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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