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.…