When an allergy-causing substance (allergen) enters the body, it usually triggers the release of histamine as part of an allergic reaction. Depending upon the location in the body, histamine can produce diverse symptoms. In the nose, it may cause the nasal tissues to secrete more mucus. Similarly, if the allergy occurs on the skin, histamine may cause skin swelling along with redness and itching. Antihistamines are drugs that oppose such actions of histamine by blocking histamine receptors on the body's cells. They are widely used to bring relief in allergies from pollen, dust and other substances. Many of them are available as over-the-counter medications.
First-Generation Antihistamines
Chlorpheniramine and diphenhydramine are useful in seasonal allergic rhinitis and a skin rash called urticaria. Like other antihistamines of the first generation, both are strongly sedative and work by blocking the histamine H1-receptor. Some other antihistamines of this generation include brompheniramine, hydroxyzine, dexchlorpheniramine and promethazine. In cough and cold medications, antihistamines are present in combination with decongestants, analgesics and/or antitussives. Though antihistamines are commonly used in allergies, your physician may need to prescribe drugs other than antihistamines if histamine release is not the main cause of your allergy symptoms.
Second-Generation Antihistamines
Second-generation antihistamines like cetirizine, terfenadine and loratidine are also prescribed for alleviating allergy symptoms like runny nose, watery and red eyes, and itchy throat. They are more selective for H1 receptors than the first-generation drugs. Because they enter the brain less readily, they cause less sedation than antihistamines of the first generation. In December 1990, the "Journal of Allergy and Clinical Immunology" published a study in which cetirizine's ability to cross the blood brain barrier was compared with that of terfenadine, hydroxyzine and dexchlorpheniramine. The researchers found that cetirizine enters the central nervous system less readily than the other three antihistamines.
Precautions
All antihistamines can cause drowsiness. Patients are therefore advised not to drive or drink alcohol while taking antihistamines. Other general side effects of antihistamines include tiredness, headache, nervousness, dry mouth, stomach pain and nausea. Also, antihistamines can interact with other drugs to increase sedation. Because children are at a higher risk for adverse effects from cold and cough medications, a survey by the Slone Epidemiology Center, published in "Pediatrics" in August 2008, has raised concern at the widespread exposure of children to these medications.
References
- "J Allergy Clin Immunol;" Cetirizine: actions on neurotransmitter receptors; A Snowman and S Snyder; 1990
- "Pediatrics;" Cough and Cold Medication Use by US Children, 1999--2006: Results From the Slone Survey; L Vernacchio et al.; 2008
