Allergy pills are common medications taken by millions of people to manage symptoms caused by the body’s reaction to allergens. When exposed to triggers like pollen or pet dander, the immune system releases a chemical called histamine, which is responsible for the sneezing, itching, and watery eyes associated with allergic responses. Antihistamines work by blocking the specific receptors that histamine acts upon, thereby preventing these uncomfortable symptoms. A persistent concern has arisen regarding a potential connection between their long-term use and changes in body weight. This concern is rooted in the fact that histamine, the very chemical being blocked, is involved in far more than just allergic reactions.
The Connection Between Allergy Pills and Weight Fluctuation
Research suggests that certain types of allergy medications are associated with weight changes, particularly when taken for extended periods. A large-scale observational study, which analyzed data from the National Health and Nutrition Examination Survey (NHANES), found a relationship between prescription H1 antihistamine use and a higher average body weight in adults. Compared to individuals who did not use these medications, those who did showed significantly greater weight, body mass index (BMI), and waist circumference.
This type of population study demonstrates a correlation, meaning that antihistamine use and higher weight tend to appear together, but it does not definitively prove causation. However, the consistent pattern across various studies indicates the link is not simply coincidental. The association suggests a biological mechanism that disrupts the body’s energy balance over time.
How Antihistamines Influence Appetite and Metabolism
The potential for weight gain stems from the way antihistamines interact with the central nervous system. Histamine acts as a neurotransmitter in the brain, originating from neurons in the hypothalamus, which regulates energy balance. In this area, histamine acts as an anorexigenic agent, meaning its normal function is to suppress appetite and signal satiety, or the feeling of fullness, primarily by stimulating the Histamine-1 receptors (H1R).
When a person takes an H1 antihistamine, the drug blocks these H1 receptors, thereby interfering with the brain’s natural appetite-suppressing signal. This disruption can lead to an increased drive to eat, often resulting in higher caloric intake. Blocking the H1R in the hypothalamus has also been shown to activate a key feeding regulator called AMP-activated protein kinase (AMPK), which further stimulates appetite and caloric consumption.
The impact is not limited to appetite, as histamine is also involved in energy expenditure and metabolism. Central histamine signaling helps accelerate lipolysis, which is the breakdown of fats for energy. When antihistamines block the H1 receptors, they may decrease the metabolic rate and reduce thermogenesis, the body’s process of generating heat and burning calories. This dual effect—increasing appetite while potentially decreasing the rate at which the body burns energy—creates an environment conducive to weight gain with long-term use.
Identifying Higher-Risk and Lower-Risk Allergy Medications
Antihistamines are broadly categorized into generations, which helps predict their likelihood of causing weight changes. First-generation antihistamines are considered higher risk because they are lipophilic, allowing them to easily cross the blood-brain barrier and strongly interact with H1 receptors in the central nervous system.
Common examples include diphenhydramine (Benadryl) and hydroxyzine. They are known for side effects like drowsiness, which can further reduce physical activity and energy expenditure. In fact, one first-generation antihistamine, cyproheptadine, is sometimes intentionally used to stimulate appetite and promote weight gain.
Newer antihistamines are considered lower risk for weight gain because they are less likely to penetrate the blood-brain barrier effectively. These non-drowsy options, such as loratadine (Claritin), fexofenadine (Allegra), and cetirizine (Zyrtec), target peripheral H1 receptors outside the brain to relieve allergy symptoms. While their central nervous system activity is reduced, some observational studies have still found an association between long-term use of certain second-generation medications, like cetirizine and fexofenadine, and increased weight or BMI, suggesting the risk is not entirely eliminated for chronic users.