Antihistamines can be valuable medicines for hives and other allergic reactions. They are sold over the counter and by prescription to fight seasonal allergies, colds and motion sickness, and as sleep aids. There are two types of antihistamines: first-generation, which causes drowsiness, and second generation, which is non-sedating for most people. Antihistamines work by blocking histamine receptors in the cells. Histamine is a tool of the immune system that fights infection by causing inflammation, swelling and an increase in mucus secretions.
Weight Gain
Antihistamines block H1 receptors in cells that trigger inflammation. Research conducted in Denmark found that histamine acts as a neurotransmitter, conducting messages between nerve cells. One of the functions of histamine is to shut off appetite, and it does this by stimulating H1 receptor cells. In addition, histamine facilitates the actions of leptin, a hormone that stops hunger signals in the brain and aids in the breakdown of fat cells. The study was published in the September 2007 issue of "Neuroendocrinology." Sedating antihistamines are also thought to contribute to weight gain by increasing inactivity, interfering with sleep patterns, and reducing the amount of calories burned.
More Senior Moments
Researchers in Indianapolis assessed the effects of antihistamines on 1,652 African Americans over the age of 70 as part of a six-year study called the Dementia Project. The class of drugs in question are called anticholinergics, which block the production of acetylcholamine, a neurotransmitter in the brain. A shortage of acetylcholamine is associated with Alzheimer's disease. The study's findings, published in the July 2010 issue of "Neurology," reported significant risk of mild cognitive impairment among African Americans who take one anticholinergic drug. Taking two medications in this class doubled the risk. In a 2005 comparison animal study of 10 antihistamines published in the "European Journal of Pharmacology," promethazine (Phenergan), desloratadine (Clarinex), diphenhydramine (Benadryl), loratadine (Claritin), and chlorpheniramine (Clor-Trimeton) were found to have the most potent anticholinergic effects, while cetirizine (Zyrtec) had less.
More Prostate Problems and UTIs
The anticholinergic effects of antihistamines are also responsible for side effects such as urinary retention and difficulty urinating. These are more likely in first-generation antihistamines such as diphenhydramine (Benadryl), chlorpheniramine (Clor-trimeton), and promethazine (Phenergan). The side effects may be particularly troublesome for men with enlarged prostates and women prone to urinary tract infections. "The New York Times" lists antihistamines among the medications that can cause lower urinary tract disorders and worsen the symptoms of benign prostatic hyperplasia.
References
- PubMed: Histamine and the Regulation of Body Weight
- PubMed: Use of Anticholinergics and the Risk of Cognitive Impairment in an African American Population
- PubMed: Comparative Anticholinergic Activities of 10 Histamine H1 Receptor Antagonists in Two Functional Models
- New York Times Health: Prostate, Enlarged
