Allergies Common Treatments Why Antihistamines Cause Weight Gain By Daniel More, MD Daniel More, MD, is a board-certified allergist and clinical immunologist. He is an assistant clinical professor at the University of California, San Francisco School of Medicine and currently practices at Central Coast Allergy and Asthma in Salinas, California. Learn about our editorial process Daniel More, MD Medically reviewed by Medically reviewed by Sanja Jelic, MD on November 08, 2019 Sanja Jelic, MD is board-certified in pulmonary disease, sleep medicine, critical care medicine, and internal medicine. She is an assistant professor and attending physician at Columbia University College of Physicians and Surgeons in New York, NY. Learn about our Medical Review Board Sanja Jelic, MD Updated on September 17, 2020 Print Antihistamines are particularly good at treating allergy symptoms such as sneezing, runny nose, and itchy, watery eyes. While antihistamines are considered a relatively safe medication, they can come with side effects, including the possibility of weight gain. Tetra Images / Getty Images What Are Antihistamines? Antihistamines are oral medications that are commonly used to treat symptoms of allergic rhinitis and allergic conjunctivitis. They work by blocking the actions of histamine, a chemical released by your body's mast cells. Our bodies need histamine. If you come in contact with an allergen, histamine helps your body get rid of it. But sometimes our bodies produce too much histamine in response to a harmless allergen, like pollen, leaving you with a runny nose and itchy eyes. That's where antihistamines can help. Antihistamines for Nasal Allergies Evidence of Antihistamine Weight Gain Older antihistamines, such as Benadryl (diphenhydramine), have well-known side effects such as drowsiness. While others, like Allegra (fexofenadine), tend to have less of these side effects. A study published in the journal Obesity found an association between the use of antihistamines and obesity. Of the almost 900 people studied, those taking antihistamines—such as Zyrtec and Allegra—were more likely to be overweight or obese than those not taking antihistamines. The reasons for this weren't clear, and it's important to note that this association doesn't mean that antihistamines directly cause weight gain. The researchers theorized that antihistamines have a similar chemical structure to certain psychiatric drugs that are known to be associated with weight gain. Antihistamines may also increase appetite, which can cause weight gain. Anecdotally, people using Xyzal (levocetirizine)—an antihistamine similar to Zyrtec (cetirizine)—they have noticed they have put on extra pounds, which is what a very small percentage of patients who used the drug during trials experienced. Older antihistamines, such as Periactin (cyproheptadine), have actually been used for the purpose of increasing appetite and weight gain in underweight children and cancer patients undergoing chemotherapy. Theories of Mechanisms If antihistamines make you drowsy, your decreased energy levels could result in less exercise and more weight gain. Alternatively, obesity is considered to be an inflammatory condition that makes a person more prone to problems such as allergies. Therefore antihistamines use is simply a marker for allergies, not the cause of the weight gain. Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Ratliff JC, Barber JA, Palmese LB, Reutenauer EL, Tek C. Association of prescription H1 antihistamine use with obesity: results from the National Health and Nutrition Examination Survey. Obesity (Silver Spring). 2010;18(12):2398-400. doi:10.1038/oby.2010.176 Couluris M, Mayer J, Freyer D, Sandler E, Xu P, Krischer J. The effect of cyproheptadine hydrochloride (Periactin) and megestrol acetate (Megace) on weight in children with cancer/ treatment-related cachexia. J Pediatr Hematol Oncol. 2008 November; 30(11): 791-797. doi:10.1097/MPH.0b013e3181864a5e