Keri M., an experienced marathon runner in her 50s, encountered significant difficulties during a recent race while using a GLP-1 receptor agonist for weight loss. Having completed numerous half and full marathons—including the 2024 Philadelphia marathon where she qualified for the Bank of America Chicago Marathon—Keri found her performance notably compromised after starting a 2.5 mg dose of tirzepatide, marketed as Mounjaro or Zepbound, less than a week before the event. She experienced severe nausea and struggled to consume the energy gels typically essential for endurance athletes, leading her to consider whether she should have begun treatment with a lower dose.
The use of GLP-1 drugs for weight loss has surged in recent years. Data from a 2026 cohort study indicated that the percentage of people taking these medications to lose weight rose from 3% to 11% over two years, with off-label use increasing fifteenfold compared to five years prior. Notably, 35% of users do not meet the BMI criteria for overweight status.
Sports dietitians such as Meghann Featherstun note the increased accessibility of GLP-1s, as prescribing criteria have loosened since their introduction. While some experts, including Heidi Skolnik, a fellow of the American College of Sports Medicine and author of a guide on GLP-1 use, suggest that longer races are possible while on these medications if one manages to maintain adequate nutrition and hydration, others urge greater caution. Skolnik emphasizes that if sufficient fueling and drinking are not achievable, continuing a race might be detrimental to health.
Side effects common to GLP-1s—nausea, appetite suppression, and gastrointestinal distress—pose challenges for endurance athletes. Some users adopt strategies like microdosing or adjusting the timing of doses to mitigate these impacts and maintain training schedules. Online communities such as the "Zepbound athletes" subreddit share advice on managing workouts and doses during GLP-1 use. However, medical researchers like Emily Lundstrom suggest that athletes aiming for peak performance should complete their race preparations before starting GLP-1 therapy, cautioning against even low-dose regimens during training.
The topic of GLP-1 use in sports has also prompted debate following high-profile endorsements. After tennis player Serena Williams became a spokesperson for a telehealth company distributing GLP-1s, discussions arose over whether these drugs promote unhealthy weight loss or help reduce stigma around their use. Many endurance runners, like Steve, a 44-year-old from Oregon, relate to the narrative of struggling to control weight despite rigorous exercise and healthy diets. Steve began using compounded tirzepatide in mid-2026 and found his running improved after reducing his dose, though his weight loss stalled.
Experts warn about potential cardiovascular effects, including increases in resting and exercise heart rates. Dr. Melanie Jay of NYU Grossman School of Medicine highlighted limited research on how second-generation GLP-1s like tirzepatide impact heart function during exercise. Other side effects typically lessen over time.
From a nutritional standpoint, Claire Shorenstein, a board-certified sports dietitian, stresses that endurance athletes are particularly vulnerable to low energy availability—a state where calorie intake fails to meet the demands of training and bodily functions. GLP-1-induced appetite suppression may exacerbate this risk, potentially leading to Relative Energy Deficiency in Sport (REDs), which can compromise bone density, immune function, and menstrual health in women.
Keri, who paused and resumed GLP-1 use during her marathon training due to adverse effects and appetite changes, stated she plans to avoid the medication until after her Chicago Marathon. Similarly, Kylie Keeton, a Texas-based runner who lost significant weight without GLP-1s but later used semaglutide and tirzepatide, faced nausea that limited her running capacity and forced her to drop out of a half marathon. Keeton experimented with dosing strategies, though medical specialists advise against altering prescribed dosing regimens, cautioning that such approaches lack clinical validation.
As GLP-1 use grows among the general public and athletes alike, experts underscore the importance of individualized assessment, careful management of side effects, and consideration of potential impacts on performance and health during endurance training.