Antihistamines side effects

Updated March 23, 2017

Antihistamines are a class of medications used to treat many kinds of allergies. As with any medication, there are certain side effects and contraindications associated with the use of antihistamines.


Antihistamines are over-the-counter medications used to treat the effects of seasonal, ongoing indoor and acute allergies. Antihistamines relieve such allergy symptoms such as runny or stuffy noses; itchy, water eyes; rashes; sneezing; and congestion.

Some well-known brands of antihistamines are Claritin, Alavert and benadryl. There are hundreds of brands of antihistamines, however. They can usually be found in the allergy section of your local pharmacy.

Antihistamines also come in different forms. The most common are tablets and capsules, but they can also be found as a liquid, which is common for use in children. Some antihistamines are also available in a nasal spray.

How Antihistamines Work

Antihistamine medications work by blocking the brain chemical histamine found within the body. Histamine is released when the body reacts to an allergen, such as dust, mould or pet dander. The release of histamine by the brain signals for your nasal passages and eyes to swell and leak to prevent the offending allergen from getting inside. However, the effects of histamine are often more bothersome than the allergen itself. By limiting your body's ability to secrete histamine, your allergy responses are thwarted.

Side Effects

Like any drug, antihistamines pose the risk for certain side effects. Common side effects reported with antihistamine use include sleepiness, headaches, dizziness, sour stomach, vision changes, dry mouth/nose and irritability. Most side effects are mild and will recede after your body adjusts to the antihistamine that you are taking.

Other side effects, such as heart palpitations, difficulty breathing, urination difficulties or trouble breathing, should be reported to your physician at once. These side effects can be indicators of an allergy to the drug itself or signs of a more serious problem.


Patients with glaucoma, ulcers of the stomach, problems getting urine to pass, high blood pressure, heart disease, seizure disorders such as epilepsy, overactive thyroid or lung disease should be strictly monitored by a physician while taking antihistamine medications. If you have any of these conditions, consult with your doctor to weigh the benefits with the risks of taking antihistamines in your particular health situation.

Pregnant and breastfeeding women should only take antihistamines under a doctor's supervision. If you are pregnant or breastfeeding, consult your physician before taking antihistamines.


Certain medications can adversely react with antihistamines. If you are currently taking sleep aids (prescription or over-the-counter), sedatives, tranquillisers, muscle relaxers, depression medications, medications for seizures or narcotic painkillers, consult your doctor before taking antihistamines.

As with all drugs, should a yellowing of the eyes or skin occur while taking antihistamines, discontinue use of the mediation and consult your doctor immediately.

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About the Author

Leeann Teagno has been writing professionally since 2006. An English major, she continues to study information systems management at American Public University. Teagno is an organic gardener, cook and technology buff with past employment in mobile communications. She also volunteers at an animal shelter and operates a home bakery.