Benadryl one a day relief tablets contain the active ingredient cetirizine, which is a type of medicine called a non-sedating antihistamine. (NB: cetirizine is also available without a brand name, ie as the generic medicine.) Cetirizine works by preventing the actions of histamine.
Histamine is a substance produced by the body as part of its defence mechanisms. It is stored in cells called mast cells, in almost all tissues of the body. When the body reacts to a foreign substance (known as an allergen, eg flower pollen), the mast cells are stimulated by the allergen and release their stores of histamine.
The released histamine then binds to its receptors (H1 receptors), causing a chain reaction that results in allergic symptoms. It causes an increase in blood flow to the area of the allergy, and the release of other chemicals that add to the allergic response.
All this results in the symptoms of an allergic reaction. In hayfever, histamine causes inflammation of the nose, eyes, skin or airways and results in itchy watery eyes, a runny nose, sneezing and nasal congestion.
Cetirizine works by blocking histamine H1 receptors. It does not prevent the actual release of histamine from mast cells, but prevents it binding to its receptors. This in turn prevents the release of other allergy chemicals and reduces the blood supply to the area, providing relief from the typical symptoms of hayfever.
Cetirizine can also be used to relieve the symptoms of a condition called chronic idiopathic urticaria. This is a chronic itchy rash, similar to nettle rash, but with no apparent cause. Blocking the actions of histamine relieves the itching and reduces the rash associated with this condition.
Cetirizine is called a non-sedating antihistamine as it does not enter the brain, and is therefore unlikely to cause drowsiness. However, some people may experience some slight drowsiness.
This medicine should not be used if you are allergic to any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.
If you feel you have experienced an allergic reaction, stop using this medicine and inform your doctor or pharmacist immediately.
Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.
Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Just because a side effect is stated here, it does not mean that all people using this medicine will experience that or any side effect.
The side effects listed above may not include all of the side effects reported by the medicine's manufacturer.
For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.
This medicine is not known to significantly affect other medicines. However, if you are already taking any other medicines you should always get advice from your pharmacist before taking this one as well, to make sure the combination is safe and appropriate.
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Cetirizine tablets and oral solutions are also available without a brand name, ie as the generic medicine.