Keri M, a marathoner in her 50s, knew the race would be terrible.This feeling couldn’t be chalked up to inexperience. Keri, who asked us to omit her full name due to the sensitive subject matter, has completed dozens of half marathons and four full marathons.She ran the 2024 Philadelphia marathon fast enough to earn a coveted 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 the first time she had raced on a GLP-1. Less than a week beforehand, she had taken her first 2.5mg standard starting dose of tirzepatide, commonly sold under the names Mounjaro and Zepbound, for weight loss.Throughout the race, Keri felt nauseous, struggling 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 that she had started at a lower dose.The number of people taking GLP-1s to lose weight has grown from 3% to 11% in the past two years. A 2026 cohort study found that 15 times more people use them for non-FDA-approved reasons compared with five years ago.
The study also found that 35% of users are not considered overweight according to the BMI chart.Graphic with three lines of text that say, in bold, ‘Well Actually’, then ‘Read more on living a good life in a complex world,’ then a pinkish-lavender pill-shaped button with white letters that say ‘More from this section’ Meghann Featherstun, an Ohio-based registered sports dietitian, has worked with multiple marathon runners who use GLP-1s; they are more accessible now than when they first came on the market and fewer clinical criteria are required for a prescription, she said.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 your 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 negatively affect running performance.
Some athletes use strategies such as microdosing to achieve their weight loss goals while still training for half and full marathons. In one subreddit, “Zepbound athletes”, users exchange advice for 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 to use a GLP-1. “Even microdosing is not a good idea,” she said.As the number of people who take GLP-1s for weight loss grows, so has the controversy surrounding these drugs.When Serena Williams, a decorated female tennis player, became a spokesperson for Ro, a telehealth company that sells GLP-1s, it opened up a debate over who should take these drugs and if they have a place in sports.
Some accused her of promoting thinness and unnecessary weight loss. Others argued that her openness may help reduce the shame surrounding 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 the age of 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 turning to 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.skip past newsletter promotionafter newsletter promotion“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.Eventually, Keeton stopped taking the drug. She had lost about 5lb in six months of taking GLP-1s and didn’t think it was worth continuing given the expense.
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 long-distance running. The experts I spoke to expressed caution over energy levels and dosage tweaks, and urged users to heed any adverse symptoms or difficulties.
The only thing that’s clear is that long-distance runners who use GLP-1s can have varying 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 lose weight. At first, Thornton noticed she struggled to run more than 5km (about 3 miles).
Eventually, 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.