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Antihistamines

Antihistamines are medicines that are commonly used to treat allergic reactions such as hay fever (allergic rhinitis). They may also be used to treat sickness, vertigo and insomnia.

Antihistamines can be used to treat a range of conditions. You might take them to ease allergic reactions. These include:

  • hay fever
  • allergic skin conditions, such as urticaria (hives) and dermatitis
  • itching
  • insect bites or stings

You may also take some types of antihistamines to help relieve the symptoms of sickness, vertigo and insomnia.

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Details

  • Types What are the main types of antihistamines?

    There are two main types of antihistamines, which are described below.

    Sedating

    The older types of antihistamines are sometimes referred to as sedating antihistamines. This type of antihistamine can affect your brain, causing you to become drowsy. An example of a sedating antihistamine is chlorphenamine maleate (eg Piriton).

    Non-sedating

    The newer types of antihistamines are sometimes referred to as non-sedating antihistamines. This type of antihistamine has less of an effect on your brain and won’t make you so drowsy. Acrivastine (eg Benadryl) is an example of a non-sedating antihistamine.

  • How they work How do antihistamines work?

    Your immune system protects you from harmful substances such as bacteria and viruses. It does this by producing antibodies which help to remove these from your body.

    In an allergic reaction, your immune system reacts to a harmless substance such as pollen, because it mistakes it for a harmful one. When this happens, a chemical called histamine is released by your immune system. Histamine is very useful because it helps damaged tissue to heal. However, this process can also cause symptoms such as:

    • a runny nose
    • itchy eyes, nose, throat and skin
    • sneezing
    • urticaria

    Antihistamines work by blocking the effect of histamine in your body. This helps to prevent inflammation and eases allergic reactions.

    If you have a severe allergic reaction, your symptoms may include:

    • difficulty breathing
    • swollen lips and eyelids
    • a fast heartbeat

    This type of allergic reaction is called anaphylaxis. If you have anaphylaxis, it’s important that you get medical help immediately as it can be very serious. You will be given an injection of adrenaline if you have anaphylaxis. Sometimes an injection of an antihistamine is used in addition to adrenaline.

  • Taking the medicine How to take antihistamines

    You can buy some antihistamines from pharmacies without a prescription. Examples include chlorphenamine maleate (eg Piriton), loratadine (eg Clarityn) and cetirizine (eg Zirtek). Some antihistamines are only available on prescription from your GP as they could have side-effects or interactions with other medicines you might be taking.

    Depending on what you need antihistamines for; you can take them as tablets, liquids, nasal sprays or a cream. Only use antihistamine creams for a short amount of time as they may cause an allergic reaction. Don’t use antihistamine creams on areas of broken skin, unless your GP has said that you can do so.

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  • Special care Special care

    If you’re pregnant, it’s recommended that you don’t take antihistamines. This is also true if you’re breastfeeding because antihistamines can get into your breast milk. If you feel you need to be treated with antihistamines while pregnant or breastfeeding, see your GP for advice.

    If you have epilepsy, you must get advice from your GP before taking a sedating antihistamine. You should not take this type of antihistamine if you have severe liver disease.

  • Side-effects Side-effects of antihistamines

    This section doesn’t include every possible side-effect of antihistamines. Always read the patient leaflet that comes with your medicine and ask your pharmacist for advice.

    The sedating forms of antihistamines will make you feel very drowsy and can affect your coordination. Therefore, it’s important that you don’t drive or drink alcohol for 24 hours after taking a sedating antihistamine. Alcohol can increase the sedative effect of these types of antihistamines. You should not drink alcohol if you’re taking these antihistamines.

    With the newer, non-sedating antihistamines drowsiness is less of a problem.

    Side-effects that are more common with the older, sedating antihistamines include:
    • headache
    • difficulty passing urine
    • dry mouth
    • blurred vision
    • feeling sick or vomiting

    Other, rare side-effects of antihistamines include:

    • low blood pressure
    • arrhythmia (abnormal heart rhythm)
    • dizziness
    • confusion
    • depression
    • disturbed sleep
    • tremor (a slight shaking of part of your body, often your hands)
    • allergic reactions (including swelling, rashes, breathing difficulties)
    • blood or liver problems

    Children and adults over 65 are more likely to get side-effects.

  • Interactions Interactions of antihistamines with other medicines

    Check with your GP or pharmacist before you take any other medicines at the same time as antihistamines.

    Tricyclic antidepressants interact with antihistamines and can increase the drowsiness side-effect. The antihistamine mizolastine can also interact with some other medicines and may cause a serious abnormal heart rhythm. This medicine is only available on prescription from your GP, who will give you advice if you need to take it.

    Some antifungal (ketoconazole) medicines and antibiotics (erythromycin) can increase the amount of non-sedating antihistamine in your body.

    It’s best not to drink alcohol while taking antihistamines of any type as this will increase the drowsiness side-effect.

  • Common names Names of common antihistamines

    Antihistamine medicines are shown in the table below. It’s important to remember that you may not be able to buy some of these medicines over the counter as they may be prescription only.

    All medicines have a generic name. Many medicines also have one or more brand name. Generic names are written in lower case, whereas brand names start with a capital letter.

    Generic names Examples of common brand names
    Non-sedating antihistamines  
    acrivastine Benadryl Allergy Relief
    bilastine Ilaxten
    cetirizine hydrochloride Benadryl Allergy Oral Syrup, Benadryl for Children Allergy Solution, Benadryl One-a-Day Relief, Piriteze Allergy, Pollenshield Hayfever Relief, Zirtek Allergy Relief
    desloratadine Neoclarityn
    fexofenadine hydrochloride Telfast
    levocetirizine hydrochloride Xyzal
    loratadine Clarityn Allergy
    mizolastine Mizollen
    Sedating antihistamines  
    alimemazine tartrate Vallergan
    chlorphenamine maleate Piriton
    clemastine Tavegil
    cyproheptadine hydrochloride Periactin
    hydroxyzine hydrochloride Atarax, Ucerax
    ketotifen Zaditen
    promethazine hydrochloride Phenergan
    For the eyes and nose  
    antazoline (with xylometazoline) Otrivine-Antistin (for eyes)
    azelastine Optilast (for eyes), Rhinoblast (for nose)
    epinastine Relestat (for eyes)
    ketotifen Zaditen (for eyes)
    olopatadine Opatanol (for eyes)
    Creams and lotions for the skin  
    diphenhydramine hydrochloride Benadryl Skin Allergy Relief Cream
    mepyramine maleate Anthisan, Wasp-Eze Bites and Stings spray, Wasp-Eze
    Travel sickness and vertigo/nausea  
    meclozine hydrochloride Sea-legs
    cyclizine Valoid
    cinnarizine Stugeron
    Sleep aids  
    diphenhydramine hydrochloride Nytol, Nytol One-a-Night
    promethazine hydrochloride Sominex
  • FAQs FAQs

    What should I do if I take too much antihistamine?

    Answer

    It’s possible to overdose on antihistamines. If you do, it’s important to tell your GP or pharmacist straight away. Go to your nearest accident and emergency department if you feel unwell. Take the medicine packet(s) with you, so your GP, pharmacist or hospital doctor can see exactly what type of antihistamine you have taken.

    Explanation

    The effects of taking too much antihistamine are similar to the side-effects of this medicine but more severe. If you take too much antihistamine, you might feel very sleepy, dizzy or confused, especially if you have taken one of the older, sedating antihistamines. Alternatively, you may feel wide awake. Other effects of taking too much antihistamine include breathing difficulties, fits and arrhythmia (abnormal heart rhythm).

    Many different medicines contain antihistamine, such as those for coughs and colds. You could easily reach the maximum safe dose if you take an antihistamine at the same time as taking one of these medicines. If you have any questions, ask your pharmacist for advice.

    What is better for hay fever - antihistamine nasal sprays or tablets?

    Answer

    The type of antihistamine that you choose to treat your hay fever (allergic rhinitis) will depend on the symptoms you have, such as whether you’re mainly sneezing or if your eyes are itchy.

    Explanation

    If you have occasional symptoms, taking antihistamine tablets when you need them may be a good first option. They might also help if you have allergic conjunctivitis or for children aged between two and five years who have hay fever. You can also use a nasal spray. This might relieve some of your symptoms quickly as it usually works within 15 minutes.

    If your symptoms are more frequent or persistent, try and stay away from what is causing your allergy, for example pollen. If you can’t do this, a steroid nasal spray or drops might help to treat your symptoms.

    If your symptoms mainly affect your eyes, your GP may prescribe you eye drops. It’s important to remember that eye drops may cause discomfort in some people.

    Talk to your pharmacist or GP if you're unsure which treatment is best for you.

    Do antihistamines become less effective if you use them for a long time?

    Answer

    You’re unlikely to develop a tolerance to antihistamines if you use them to treat allergies, even with daily use. Tolerance means you will need to take higher doses of the medicine for it to have the same effect.

    Explanation

    Some studies have shown that if you use antihistamines daily to prevent symptoms of hay fever and other allergies, they may have the same effectiveness even after a long period of taking them.

    The same isn’t true of the antihistamines that are used to treat insomnia, such as promethazine hydrochloride (eg Phenergan). The sedative effect of antihistamines reduces over time. If you have long-term insomnia, see your GP for advice. This is because all medicines for insomnia should only be used in the short term.

    If you’re finding that your symptoms aren’t well controlled by the antihistamine treatment you’re using, speak to your GP or pharmacist. It might be that you’re not taking the medicine in the most effective way. For instance, if you’re using an antihistamine nasal spray, it’s possible that you’re not applying it correctly. Or you may find that you need additional treatment, such as a steroid nasal spray.

  • Resources Resources

    Further information

    Sources

    • Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press. www.medicinescomplete.com, accessed 17 March 2014 (online version)
    • Antihistamines. PatientPlus. www.patient.co.uk/patientplus.asp, published 19 January 2012
    • Concise colour medical dictionary. 3rd ed. Oxford: Oxford University Press, 2002:30, 39, 319, 343, 344, 693 (printed version)
    • Overview of allergy and atopy. The Merck Manuals. www.merckmanuals.com, published August 2013
    • Hay fever and allergic rhinitis. Allergy UK. www.allergyuk.org, published December 2013
    • Allergy medications. Allergy UK. www.allergyuk.org, published July 2013
    • Day & Night Nurse capsules. electronic Medicines Compendium. www.medicines.org.uk, published 10 February 2014
    • Piriton allergy tablets. electronic Medicines Compendium. www.medicines.org.uk, published 30 November 2011
    • Antihistamines. DermNet NZ. www.dermnetnz.org, published 13 March 2014
    • Cough, cold, and allergy preparation toxicity. Medscape. www.emedicine.medscape.com, published 16 September 2013
    • Allergic rhinitis. NICE Clinical Knowledge Summaries. cks.nice.org.uk, published November 2012
    • Conjunctivitis - allergic. NICE Clinical Knowledge Summaries. cks.nice.org.uk, published August 2012
    • del Cuvillo A MJ, Bartra J, et al. Comparative pharmacology of the H1 antihistamines. J Investig Allergol Clin Immunol 2006; 16(1):3–12

  • Related information Related information

  • Author information Author information

    Reviewed by Kuljeet Battoo, Bupa Health Information Team, June 2014.

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