Antihistamines are a group of medicines which act by blocking the action of the chemical called histamine in the body. Either H1 or H2 histamine receptors can be blocked by medicines, but the group commonly known as antihistamines blocks the H1 receptor. They have a number of uses, but are most often used to treat allergies.
What are antihistamines used for?
If you are one of the many people who get hay fever each year, you will know all about antihistamines. This is one of the most common uses of antihistamines. They can also be used for various other problems. For example:
- Hives (urticaria).
- Itching (pruritus).
- Feeling sick (nausea) and being sick (vomiting).
- Severe allergic reactions (anaphylaxis).
Where can I get them?
Some of these medicines are available to buy over the counter from your pharmacist. Others are only available on prescription.
How do I take them?
These medicines come in a variety of forms, as mentioned above. Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often. Read the leaflet that comes with your particular brand for further information.
How quickly do antihistamines work?
An antihistamine tablet typically starts to work within 30 minutes after being taken. The peak of effectiveness is typically within 1-2 hours after being taken.
How long is treatment needed?
This can vary depending on the reason for treating you. If you have hay fever you may take the medicine throughout the pollen season.
How effective are they?
Usually it is possible to find an antihistamine which keeps your symptoms controlled. Sometimes you may need to experiment and try more than one to find the one that works best for you. For hay fever, sometimes if the antihistamine tablet isn't enough by itself, you may need eye drops and/or a nasal spray as well to completely sort out all your symptoms.
Antihistamines are generally more effective when taken constantly rather than intermittently. This is particularly applicable for people with hay fever (seasonal allergic rhinitis). In the summer months the pollen count is generally higher and you may be in contact with the allergen on a regular basis. Taking the medication regularly may help keep your symptoms under control. Their effectiveness will also depend on the dose you take and what form the medicine is given in.
Can I take them if I am pregnant or breast-feeding?
Drug companies are naturally pretty reluctant to go around testing medicines on pregnant or breast-feeding women, so there aren't any studies to guide them. They tend to advise against to be on the safe side, although there is no evidence they cause a problem. Discuss this with your doctor who will talk you through the options. If the benefits of treatment are thought to outweigh any possible risks, the one usually advised is loratadine.
Further reading and references
British National Formulary; NICE Evidence Services (UK access only)
Allergic Rhinitis; NICE CKS, October 2015 (UK access only)
Urticaria; NICE CKS, May 2016 (UK access only)
Guidelines for the management of allergic and non-allergic rhinitis; British Society for Allergy and Clinical Immunology (January 2008)
Primary Care Rhinitis algorithm; British Society for Allergy and Clinical Immunology (BSACI)
Over-the-counter cough and cold medicines for children; Medicines and Healthcare products Regulatory Agency (MHRA), 2009
Hi! I have a number of food allergies and really do my best to avoid triggers. A few weeks ago I got the worse rash and hives I have ever had. I went to a walk in clinic and they gave me a cortisol...maddysmom2015
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