Struggling to finish that workout? Your fatigue may be tied to sleep quality, not necessarily how long you snoozed the night prior. (Photo by BLACKDAY on Shutterstock)
In a nutshell
- Taking antihistamines before exercise cuts fitness improvements in half compared to exercising without them
- Histamine released during workouts helps trigger beneficial adaptations in muscles and blood vessels
- The study used much higher doses than typical allergy medications, but suggests timing antihistamine use around workouts may matter
EUGENE, Ore. — If you pop an Allegra or Claritin before hitting the gym to avoid seasonal sniffles, you might want to think twice. New research reveals that common over-the-counter antihistamines could be blocking your body’s ability to get fitter and more resilient from exercise.
Scientists at the University of Oregon discovered that people who took antihistamines before every workout gained only half the fitness improvements compared to those who exercised without the medications. The findings suggest that histamine, the same chemical that makes us sneeze and itch, may actually be essential for unlocking the full benefits of training.
The Surprising Role of Histamine in Exercise
Inside our muscles, histamine appears to act as a master regulator of fitness adaptations. During exercise, muscle tissue naturally releases histamine. Rather than signaling an allergy, this release triggers positive physiological changes. Histamine binds to receptors in blood vessels and muscle cells, improving circulation, enhancing nutrient delivery, and kickstarting the cellular processes that help the body adapt to physical activity.
In the study, 16 healthy adults were randomly assigned to take either antihistamine pills or placebo capsules before each of 21 supervised cycling sessions over six weeks. By the end of the program, the placebo group had improved their peak power output by 18%. The antihistamine group? Only 9%.
“These results indicate that exercise-induced histamine release is important in generating many of the positive adaptations to exercise training that result in improvements in fitness,” the researchers concluded.
What Actually Gets Blocked When You Take Antihistamines
The researchers used two antihistamines: 540 milligrams of fexofenadine (commonly sold as Allegra) and 300 milligrams of ranitidine (formerly available as Zantac). These doses were significantly higher than standard over-the-counter recommendations, and were chosen specifically to block more than 90% of histamine activity during exercise.
By measuring various physiological markers, the team revealed what histamine actually does during a workout.
Metabolic adaptations were dramatically affected. The placebo group increased their muscle’s ability to burn fuel (measured by the enzyme succinate dehydrogenase) by 136%. The antihistamine group showed only a 3% increase. This enzyme plays a crucial role in aerobic energy production during exercise.
Vascular improvements were also muted. Normally, exercise enhances blood flow to working muscles. In this study, peak blood flow improved by 45% in the placebo group compared to just 25% in those taking antihistamines.
Both groups improved their maximum oxygen uptake (VO₂peak) by similar amounts: about 13% in the placebo group and 9% in the antihistamine group; but the cellular and metabolic adaptations that support long-term health and performance were clearly impaired by blocking histamine.
Real-World Implications for Your Workouts
It’s important to note that the study used much higher doses than what people typically take to manage allergies. For example, a standard Claritin tablet contains just 10 milligrams of loratadine. However, the researchers intentionally used high doses to ensure complete receptor blockade during the workout window.
Participants only took the antihistamines one hour before exercise and not on rest days. The disruption occurred specifically during training, when histamine levels rise in response to physical effort.
Previous studies have observed similar effects with high-intensity interval training, suggesting that histamine’s role in fitness adaptations extends across different types of exercise, not just endurance cycling.
Finding the Right Balance for Allergy Sufferers
This research doesn’t suggest that people with allergies should tough it out through workouts with itchy eyes and stuffed noses. Instead, it highlights the need for strategic thinking about how and when to use antihistamines.
Some potential approaches include adjusting the timing of allergy medications to avoid peak workout hours, using the lowest effective dose, or selecting drugs that target only one type of histamine receptor. The study used medications that blocked both H1 and H2 receptors, which may not reflect typical allergy treatment.
And while ranitidine was used in the study, it has since been voluntarily withdrawn from the market in many countries due to unrelated safety concerns. Still, the principle remains: completely blocking histamine during exercise may interfere with your body’s ability to adapt.
The Bottom Line
The next time you reach for an antihistamine before your workout, consider whether it’s absolutely necessary. While this study used higher doses than typical allergy medications, it demonstrates that histamine isn’t just an annoying byproduct of immune reactions, rather it’s an active participant in helping your body adapt to exercise.
For serious athletes or anyone working hard to improve their fitness, the message is clear: your body’s natural responses to exercise, even the uncomfortable ones, might be more important than previously thought. Sometimes the best approach is to let your body do what it’s designed to do, sniffles and all.
Paper Summary
Methodology
Researchers conducted a randomized, double-blind, placebo-controlled study with 16 healthy, non-smoking adults between ages 18-40. Participants were randomly assigned to receive either antihistamine medications (540mg fexofenadine plus 300mg ranitidine) or matching placebo capsules one hour before each exercise session. The six-week program included 21 supervised exercise sessions consisting mainly of moderate-intensity cycling with some high-intensity intervals. Researchers measured fitness improvements, blood vessel function, muscle metabolism, and muscle tissue changes through various tests including peak exercise tests, ultrasound measurements, blood volume analysis, and muscle biopsies in a subset of participants.
Results
The antihistamine group showed dramatically reduced fitness improvements compared to the placebo group. Peak power output increased by 18% in the placebo group versus only 9% in the antihistamine group—a rate of improvement of 3.05% per week versus 1.62% per week respectively. Muscle oxidative enzyme activity increased 136% in the placebo group but only 3% in the antihistamine group. Blood vessel function improvements were also blunted, with peak reactive hyperemia increasing 45% in placebo versus 25% in antihistamine groups. However, VO2max improvements were similar between groups, increasing about 13% and 9% respectively.
Limitations
The study had a small sample size of only 16 participants and was further impacted by drug recalls and pandemic restrictions during data collection. Some measurements were limited to even smaller subsets of participants. The antihistamine doses used were much higher than typical over-the-counter allergy medications. The study duration was relatively short at six weeks, and participants were young, healthy individuals, so results may not apply to other populations or longer training periods.
Funding and Disclosures
This research was supported by National Institutes of Health Grants AG072805 and HL115027, American Heart Association Grant 17GRNT33660656, and the Eugene and Clarissa Evonuk Memorial Graduate Fellowship. The authors declared no conflicts of interest, financial or otherwise.
Publication Information
This study was published in the Journal of Applied Physiology, volume 138, pages 1398-1410, in 2025. The paper was titled “Histamine is a molecular transducer of adaptation to endurance exercise training in humans” and was authored by Dylan C. Sieck and colleagues from the University of Oregon and Mayo Clinic.
