• Antihistamines: Patient Information

    What is histamine?

    Histamine is a protein that is used by the immune system to help protect the cells of the body from infection.If your immune system detects the presence of a harmful foreign object, such as bacteria, or a virus, it will release histamine into nearby cells.The histamine causes small blood vessels to expand and the surrounding skin to swell. This is known as inflammation.


    Why would I take antihistamines?

    You might take an antihistamine if you have symptoms of allergies, such as:hay fever, allergic rashes, and itchy skin. Certain antihistamines block the histamine receptors in the brain. These can be useful as a treatment for travel sickness, vertigo or sickness related to migraine.


    What are the main types of antihistamines?

    There are two main types of antihistamines: sedating and non-sedating.

    Sedating antihistamines work in your brain as well as the rest of your body, and they can cause sleepiness. Examples include chlorphenamine.Non-sedating antihistamines don’t pass into your brain so easily, so they don’t make you sleepy. This also means that they don’t relieve sickness. Examples include cetirizine and desloratadine.



    What type of antihistamine should be used?

    Second generation antihistamines are usually recommended, although there may be some circumstances where the drowsiness that is caused by first generation antihistamines can be of some use. For example, if you are having problems sleeping because you have itchy skin.


    In what forms antihistamines are available?

    Antihistamines are available in tablet form, in the form of creams, and as a nasal sprays.


    What are the main indications to use antihistamine?

    Allergic conditions that can benefit from the use of antihistamines include:

    ▪   Allergic rhinitis

    ▪   Atopic eczema

    ▪   Urticaria

    ▪   Allergic conjunctivitis

    ▪   Allergic reactions that are caused by insect bites

    ▪   Allergic reactions that are caused by food allergies.


    Can I take antihistamine during pregnancy?

    As a general rule you should avoid taking any medication during pregnancy unless there is a clear clinical need. You should also always check with your doctor before taking any medication. If your doctor feels that you need antihistamine medication, then the antihistamines thought to be safest to use during pregnancy arecetirizine, loratidine or chlorphenamine.


    Who should not get antihistamines?

    ▪   Sedating antihistamines must be used with caution in:

    Benign prostatic hyperplasia and urinary retention

    Acute glaucoma

    ▪   Epilepsy: use with caution.

    ▪   Hepatic and renal impairment.

    ▪   Pregnancy: antihistamines should be prescribed only where absolutely necessary in pregnant patients; avoid in first trimester. Long-term clinical experience suggests that the older agent chlorphenamine is suitable for use in pregnancy.


    What are the main side effects of antihistamines?

    ▪   Sedation – This is a problem with many 1st-generation antihistamines but may also affect some patients taking 2nd-generation products. Drowsiness tends to diminish over time.

    ▪   Less common side-effects, mainly from sedating antihistamines, include:headaches, difficulty in passing urine, dry mouth, blurred vision, vomiting, constipation or diarrhea, skin or eye irritation (from creams, lotions and eye drops).

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