Introduction

Antihistamines are drugs which treat allergic rhinitis and other allergies. Typically people take antihistamines as an inexpensive, generic, over-the-counter drug that can provide relief from nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with few side effects. Antihistamines are usually for short-term treatment. Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection. Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use.

Antihistamines also may be used to treat motion sickness, insomnia (difficulty sleeping), and anxiety. Antihistamines work by blocking the effects of chemical called histamine that is responsible for many allergic symptoms.

Patients who experience significant allergic symptoms regularly may take daily antihistamines to keep their symptoms under control. Antihistamines also can be used on an as needed basis for those who experience occasional symptoms or symptoms triggered by exposure to certain irritants such as animal hair, plants, medications, and food products. Some antihistamines also may be used occasionally to help with sleep.

Types of Antihistamines

Many different brands and forms of oral antihistamines are available over the counter (OTC). Oral antihistamines are available as pills, chewable tablets, orally disintegrating tablets, capsules, and liquid. Some antihistamines are found in combination pills that contain other medications. For example, antihistamines are commonly combined with decongestants (for example, Claritin-D, Zyrtec-D, Allegra-D), a class of medicine that is used to dry up the nasal passages and relieve head congestion.

Antihistamines are divided into two categories, first generation or older agents and second generation or newer agents. First generation antihistamines have many drawbacks including side effects of drowsiness and significant anticholinergic side effects that, for example, can cause difficulty urinating or constipation, and are therefore not used very often. The newer antihistamines (second generation) are less likely to cause these side effects.

Antihistamines also differ in how long they work. Long-acting antihistamines provide    symptom relief for up to 8-12 hours, while shorter acting agents last for up to 4 hours but begin working faster.

Side Effects of Antihistamines

First generation antihistamines are used less often to treat allergies because they cause significant sedation. First generation antihistamines also should be used cautiously in older adults as they are more susceptible to their anticholinergic side effects including drowsiness, dry mouth, decreased tear production, urinary retention (trouble urinating), blurred vision, constipation, and agitation.

Second generation antihistamines are less sedating than their first generation counterparts. Cetirizine can be sedating for some patients at normal recommended doses while sedation seems to only be a concern with loratadine at higher than normally recommended doses. Fexofenadine is the least sedating. The side effects of second generation antihistamines are: dizziness, dry mouth, dry eyes, blurred vision, problems urinating, constipation, mental disturbances.

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Regards

Mary Wilson

Editorial office

Clinical Pharmacology and Toxicology Research

E-mail: pharmatoxicol@eclinicalsci.com