The study considered 89 ultramarathoners split into two groups — one that took 3-4 doses of 400-milligram ibuprofen (that’s two Advil pills) every four hours toward the end of the race and one that took a placebo (via Harvard Health). The races lasted seven days and the athletes started taking ibuprofen on the fifth day.
What researchers found was that the group that took the NSAID showed more evidence of kidney injury at the end of the race (over half of them showed signs). But this doesn’t mean that there was no stress on the kidneys for those who took the placebo (one-third of them showed signs). In fact, 44% of all these endurance athletes had significant kidney injury when the race was done.
However, the fact that the study was done on ultramarathoners makes it hard to apply to everyone who engages in some form of physical exercise. Most of us don’t push our bodies to this limit. “The potential kidney side effects of NSAIDs are specific to endurance/cardiovascular exercise in which blood flow is being pulled away by multiple other systems,” explained professor of physical therapy at the Indiana University School of Health and Rehabilitation Sciences, Stuart Warden (via SELF). So, while popping an Advil might be unsafe for serious endurance athletes, the same cannot be said for recreational exercisers who occasionally take ibuprofen before a workout. Endurance training in itself (Advil or not) can be hard on your kidneys, per experts. Adding an NSAID into the mix makes it worse.