Most of us don’t think twice about popping an antihistamine when seasonal allergies kick in—runny noses and itchy eyes are hard to ignore, especially while doing a tough training session. New research suggests that if you’re training consistently and not seeing the fitness gains you expect, it might be worth looking at what is in your medicine cabinet.
A recent study out of the University of Oregon, published in the Journal of Applied Physiology, suggests that over-the-counter allergy medications could be interfering with how your body adapts to endurance training. Here’s what you need to know.
More than just allergies
Histamine is often viewed as the culprit behind allergic reactions, but it turns out it plays a much larger role and is essential in recovery and adaptation after exercise. A chemical compound that is key to your body’s inflammatory response system, it is also released whenever your body thinks a harmless substance, like pollen, is a threat, and can cause the runny nose and sneezing you might be familiar with as allergies.
When you train, your muscles undergo small-scale damage that triggers the body’s repair processes. Histamine helps regulate blood flow and immune response in this recovery window, acting almost like a messenger that signals where help is needed.
Researchers wanted to understand what happens when that process is interrupted. So they designed a six-week training study, splitting participants into two groups: one took a combination of common antihistamines after each workout, while the other did not. The group that blocked histamine responses showed significantly fewer improvements in fitness.
The same work with slower gains
Both groups completed 21 endurance sessions over six weeks. When it came to performance markers like peak power and muscle efficiency, the results were noticeably different: those who took antihistamines saw only about half the improvement in aerobic power compared to the control group. And their muscle tissue didn’t show the same increases in mitochondrial enzyme activity, which is key for endurance and energy production.Even though both groups trained the same amount, the bodies of those who took antihistamines didn’t adapt as well.
What this means for runners
This doesn’t mean you need to suffer through hay fever to keep your VO₂ max intact. But it does suggest that taking high doses of antihistamines—especially after workouts—could limit your training return. The researchers used a larger-than-normal dose to test the mechanism, but it raises important questions for everyday use, especially for runners who train regularly through allergy season.
It may come down to finding a balance: using the minimum effective dose to manage symptoms and avoiding unnecessary use around workouts. While more research is needed to know how smaller or intermittent doses affect training, it’s clear that histamine isn’t just a nuisance molecule—it might be helping you get fitter.
