Antihistamines are a group of medications that prevent the level of histamine from rising in our body and reduce the symptoms such as itching that this can cause. Antihistamines can be used to treat a range of conditions, including allergic reactions, motion sickness, and insomnia.
Antihistamines that are available to buy over the counter in the US include chlorpheniramine, diphenhydramine (Benadryl), loratadine (Claritin), cetirizine (Zyrtec), levocetirizine (Xyzal) and fexofenadine (Allegra). These are generally licensed for treating hay fever, allergic reactions to pets, dust mites and food, insect bites, chickenpox rash, and prickly heat rash. They are available as topical and oral formulations.
If you are suffering from hay fever or allergic-type symptoms such as an itchy rash or bite on the skin or redness and minor skin swelling, then an antihistamine may help relieve these symptoms for you.
Minor allergic reactions to products (washing detergent, creams, perfume) and food or plant reactions (stinging nettles) can be safely treated with over-the-counter antihistamines. They should get better in hours to days. Make sure to avoid whatever causes the reaction in the future.
If you find that you are getting regular allergic reactions and do not know why, you should discuss it with your doctor, and you may benefit from getting allergy testing.
Histamine is a naturally occurring chemical in your body that is activated and released from your cells when a foreign substance triggers your immune system. Histamine sets off a chain reaction, allowing your immune system to detect and remove the thing that caused it.
While a useful and important response, the release of histamine causes redness, swelling, and itchiness that can be bothersome. Individuals that suffer from hay fever or show allergic sensitivity to harmless stimuli have a higher-than-normal amount of histamine released by the immune system.
Antihistamines help prevent and reduce histamine's effect in the body and therefore are effective at relieving allergic symptoms.
Antihistamines can be classified as drowsy or non-drowsy. Chlorpheniramine and diphenhydramine can make you drowsy. Care should be exercised when driving or operating heavy machinery, and alcohol should be avoided. Some antihistamines that cause drowsiness, such as diphenhydramine, are also found in over-the-counter sleeping tablets.
Some other over-the-counter antihistamines are classed as non-drowsy: cetirizine (Zyrtec), levocetirizine (Xyzal), loratadine (Claritin) and fexofenadine (Allegra). These are less likely to cause drowsiness, but it is still a possible side effect.
Different medications have different doses and schedules for how often you can take them. Check with your pharmacist or read the box carefully if unsure.
Most antihistamines are safe for long periods, for example, throughout hay fever season. However, chlorphenamine should not be used for two weeks without consulting a doctor. Antihistamine creams are also available and may be more suitable for treating localized reactions such as insect bites.
Talk to your doctor if you are pregnant or breastfeeding before taking antihistamines. Do not take antihistamines three days before taking an allergy skin test, as this can affect your results. Do not take drowsy antihistamines if you have breathing problems, glaucoma, or an enlarged prostate.
Like all medications, don’t take it if you have previously had an allergic reaction to the medication, if you have severe kidney or liver problems, or if you have other medications or medical conditions that you would normally discuss with your doctor or pharmacist before starting something new.
Common side effects include drowsiness. This is more common with chlorpheniramine and diphenhydramine but may also be experienced with non-drowsy antihistamines. Other side effects include dizziness and headache.
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