Drugs classified as antihistamines include prescription medications such as Atarax, Clarinex, Palgic and Xyzal and over-the-counter medications such as Tavist, Zyrtec, Claritin and Allegra, WebMD states. Antihistamines inhibit the functioning of histamines and are typically used to alleviate allergy symptoms, Drugs.com explains.
Antihistamines reduce allergy symptoms by blocking histamine reception, WebMD notes. Allergy symptoms, such as itching or swelling, are caused by histamines stimulating the appropriate receptors. The body creates histamines after coming into contact with the substance it is allergic to, such as dust mites or pollen. Antihistamines successfully treat diverse allergy symptoms, but they are ineffective at dealing with certain symptoms such as nasal congestion. Because of this, some medications include both an antihistamine and a decongestant.
Antihistamines may cause a range of side effects including dizziness, nausea, drowsiness, inability to urinate, blurry vision and dryness in the mouth, WebMD notes. Antihistamines referred to as first-generation antihistamines may cause more side effects than newer-generation antihistamines and are particularly likely to cause drowsiness. Individuals taking a drowsiness-causing antihistamine should avoid taking it before driving or operating heavy machinery.
The presence and severity of antihistamine side effects may be affected by other medications and conditions, according to WebMD. Patients with conditions such as glaucoma, kidney disease, heart disease, elevated blood pressure and liver disease should consult their physician before taking antihistamines. Pregnant and nursing women should also consult their physician.