Keri M, a marathoner in her 50s, knew the race would be rough. This wasn’t due to inexperience. Keri, who asked us to withhold her full name because of the sensitive topic, has run dozens of half marathons and four full marathons. She ran the 2024 Philadelphia Marathon fast enough to earn a spot in this year’s Bank of America Chicago race, one of the eight world marathon majors. But this half marathon felt different. It was her first race while on a GLP-1. Less than a week earlier, she had taken her first 2.5mg starting dose of tirzepatideโsold under the brand names Mounjaro and Zepboundโfor weight loss. Throughout the race, Keri felt nauseous and struggled to keep down the energy gels many endurance runners rely on for fuel. “After that half marathon, I was so sick,” she said. Keri had a “severe” reaction to the 2.5mg dose and, in hindsight, wishes she had started at a lower dose.
The number of people taking GLP-1s for weight loss has grown from 3% to 11% over the past two years. A 2026 cohort study found that 15 times more people use them for non-FDA-approved reasons than five years ago. The study also found that 35% of users are not considered overweight according to the BMI chart.
Meghann Featherstun, an Ohio-based registered sports dietitian, has worked with several marathon runners who use GLP-1s. She said the drugs are more accessible now than when they first came on the market, and fewer clinical criteria are required for a prescription. Some experts, such as Heidi Skolnik, a fellow of the American College of Sports Medicine and author of Your GLP-1 Game Plan, believe it’s possible to run longer races while taking GLP-1s. “If you can eat enough to fuel your workouts and runs and drink enough, then go for it. Good for you. But if you can’t, then don’t. Then you’re working against your health and wellbeing,” she said.
GLP-1 side effects such as nausea and loss of appetite can hurt running performance. Some athletes use strategies like microdosing to reach their weight-loss goals while still training for half and full marathons. In one subreddit, “Zepbound athletes,” users swap advice on training and performance while on the drugs, such as skipping doses and how to get enough fuel for races. Some researchers, such as Emily Lundstrom, an assistant professor of clinical physical therapy who studies low energy availability, believe endurance athletes focused on performance should wait until they have finished preparing for a race before using a GLP-1. “Even microdosing is not a good idea,” she said.
As the number of people taking GLP-1s for weight loss grows, so has the controversy around these drugs. When Serena Williams, a decorated tennis player, became a spokesperson for Ro, a telehealth company that sells GLP-1s, it sparked a debate over who should take these drugs and whether they belong in sports. Some accused her of promoting thinness and unnecessary weight loss. Others argued that her openness could help reduce the shame around GLP-1s. Yet despite the pushback, runners relate to her journey.
Steve, a 44-year-old from Oregon who asked to be identified by only his first name because of the deeply personal topic, has always considered himself active. Even when he wasn’t training for a long-distance race, he went for casual runs, boxed, and lifted weights. Around age 41, he started gaining weight. “No matter how hard I trained, no matter how much I tracked my macros and stayed on plan, I just kept gaining weight,” he said. He noticed Williams had a similar story of turnShe started a GLP-1 because she couldn’t reach her desired physique through diet and exercise. Steve started a tirzepatide compound in July 2026 and soon began training for an October half marathon. Initially, runs didn’t feel great. Five miles usually came easy, but now required multiple breaks and spiked his heart rate higher than usual. Steve cut his 2.5mg dose in half after four weeks, a tip he saw on Reddit. He can now train without a heart-rate spike. “I don’t feel as exhausted any more,” he said. His weight loss has stalled, but he said it wouldn’t be worth taking the drug if he couldn’t train properly on it. Dr Melanie Jay, a professor at the NYU Grossman School of Medicine, said GLP-1s can cause a two to 10bpm increase in resting heart rate, but further increases are possible during exercise. However, Jay noted that there weren’t many studies on how the second generation of GLP-1s (such as the tirzepatide Steve takes) affect exercise and heart rate. Other common side-effects of GLP-1s are gastrointestinal, including nausea and constipation, which Jay said can improve with time. Long-distance runners should exercise caution when it comes to these drugs, said Claire Shorenstein, a board-certified sports dietitian. “Typically, I would not want someone to be on a GLP-1 while actively training for a marathon or any other type of endurance event,” she said. Because marathoners and other endurance athletes are already at risk for underfueling, Shorenstein says taking a GLP-1, which suppresses appetite, can lead to a chronic state of low energy availability, where the body doesn’t have enough energy left for “all the other functions that our body needs so that we can stay alive and be healthy”. Low energy availability is not permanent, but over time, it can cause Relative Energy Deficiency in Sport (Reds), which may reduce bone density and decrease immunity. In women, it can cause hypothalamic amenorrhea (loss of the menstrual cycle). Reds is treated through adequate nutrition and exercise adjustments. “I was definitely at risk for Reds because I was underfueling,” said Keri, the runner who was training for the 2026 Chicago Marathon. She has stopped and restarted her GLP-1 use during training, due to harsh side-effects, weight gain and appetite loss. “I’m not doing another dose until after the marathon,” she said. Kylie Keeton, a 32-year-old based in Texas, lost 170lb without a GLP-1. However, when she increased her running mileage, she noticed the number on the scale began to rise. Despite feeling “super embarrassed” about it, she decided to try semaglutide at the end of 2025 and eventually switched to a tirzepatide. But side-effects such as nausea made it hard to hit her mileage, and she had to drop out of a half marathon. Keeton felt torn about whether to continue taking the drug. It was hard to decide whether weight gain or not being able to run was worse, she said. Keeton tried various strategies to integrate GLP-1 use with her fitness practices, such as breaking up her weekly dose into two smaller doses. But this is not recommended because “it is not how the drug was studied in any of the clinical trials”, said Dr Nidhi Kansal, who specializes in GLP-1 treatments at Northwestern Medicine. Kristy Baumann, a Minnesota-based registered dietitian who works with endurance runners, said she doesn’t personally see clients take doses in this way. However, she notes athletes should time their doses after a long run or race day: if you take it the day before, “you’d have a greater chance of GI issues”, she said. Cravings ‘like heroin withdrawal’: what happens when weight-loss drugs don’t work? Read more Eventually, Keeton stopped taking the drug. She had lost about 5lb in six months ofShe was taking GLP-1s but didn’t think it was worth continuing because of the cost. Now she can run without feeling nauseous and hopes to improve her personal record by 45 minutes at the 2026 BMW Dallas Marathon. Overall, there is little research on using GLP-1s while running long distances. The experts I spoke to were cautious about energy levels and dosage adjustments, and they urged users to pay attention to any adverse symptoms or difficulties. The only thing that’s clear is that long-distance runners who use GLP-1s can have very different experiences. Nikki Thornton, a 47-year-old marathon runner based in Taree, Australia, started a GLP-1 in 2023 to lower her blood sugar levels, which were reaching pre-diabetic ranges, and to lose weight. At first, Thornton found she struggled to run more than 5km (about 3 miles). Eventually, she completed the sweltering Sydney Marathon but experienced gastrointestinal issues. “It was the worst one I’ve ever done,” she said. About six months after starting a GLP-1, she said her body adjusted. In 2025, she improved her Sydney Marathon time by three hours. While Featherstun, the sports dietitian from Ohio, works with marathon runners on GLP-1s, she said they rarely stay on them for long. “Every single person who’s been on it, I’ve got an email from them. ‘I’m done with it. I’m off it. I can’t handle this.’ So I do think it’s a really tricky combination,” she said.
Frequently Asked Questions
FAQs Running a Marathon While on a GLP1
1 What is a GLP1 in simple terms
GLP1 drugs are medications that mimic a gut hormone to curb appetite slow digestion and help control blood sugar Theyre used for diabetes and weight loss
2 Can I run a marathon while taking a GLP1
Possibly but only with medical clearance and careful planning Its not automatically offlimits but it raises real risksespecially around fueling and hydrationso talk to your doctor first
3 Is it a bad idea to run a marathon while on a GLP1
It can be risky if youre not prepared The main dangers are low blood sugar dehydration and not getting enough calories during a 262mile race With a solid plan and medical OK many people do it safely
4 Why is fueling harder on a GLP1
These drugs slow stomach emptying and kill appetite That makes it hard to eat the carbs and fluids you need before and during a marathonand eating too much too fast can cause nausea
5 Whats the biggest risk during the race
Underfueling Runners on GLP1s often cant stomach enough carbs which can lead to hitting the wall dizziness or bonking Low blood sugar is a particular concern if you also take diabetes medication
6 Can a GLP1 cause dehydration while running
Yes indirectly Reduced appetite means you may drink less and nausea or vomiting can worsen fluid loss Dehydration raises the risk of cramps heat illness and poor performance
7 Should I stop my GLP1 before race day
Only if your doctor says so Some doctors adjust or pause the dose before a big race but never change it on your ownthe decision depends on your health and the medication
8 How do I practice fueling for race day
Test everything on long training runs gels chews sports drinks and timing Find what your stomach tolerates at race pace and rehearse your prerace meal too
9 What should I eat before the race
Something light carbfocused and easy to digestlike a banana
