The common practice of taking ibuprofen for pain relief while competing in ultramarathons causes an increased risk of acute kidney injury, a Stanford study says. People who take the painkiller ibuprofen while running very long distances double their risk of acute kidney injury, according to a study by researchers at the Stanford University School of Medicine and several other institutions.

As many as 75 percent of ultramarathoners use the nonsteroidal anti–inflammatory drug, or NSAID, in this fashion, according to Grant Lipman, MD, clinical associate professor of emergency medicine at Stanford and director of Stanford Wilderness Medicine. And while most cases of acute kidney injury appear to resolve spontaneously, the condition has the potential to progress to renal failure, he said.

Lipman is lead author of the study, which was published online July 5 in Emergency Medical Journal. Brian Krabak, MD, a sports and rehabilitation medicine specialist at the University of Washington–Seattle, is the senior author.

“In medical school, we were all taught to be careful of ibuprofen because it decreases blood flow to the kidneys,” he said. However, almost all previous studies looking at the effect of the drug on the kidneys in running events have shown no negative effects, he said.

Lipman and his colleagues conducted the first randomized, placebo–controlled, double–blinded study to test the use of ibuprofen in ultramarathoners. They hypothesized that ibuprofen would not result in an increased rate of acute kidney injury compared to placebo.

The 89 participants who completed the trial were randomized to take either ibuprofen or a placebo during a 50–mile section of one of four different seven–day, 155–mile ultramarathons. They were required to refrain from taking ibuprofen at least 12 hours prior to the 50–mile section. They ran in ultramarathons either in China, Chile, Ecuador or Sri Lanka. They ran through wilderness terrain with few roads and varying topography, and they carried all their personal items for the duration of the race, including all their gear, food and clothing.

The morning of the 50–mile section, the participants were weighed, and each was given a baggie of either sugar pills or 400–milligram ibuprofen pills. They were told to take one pill every four hours.

Twelve– to 36–hours later, depending on the speed of the runners, the participants were met by the researchers at the medical tent. There, they were weighed and their electrolyte levels and renal functioning were measured.

Forty–seven percent of the participants took ibuprofen, and 53 percent took the placebo. Results showed that about 39 of the 89 participants had acute kidney injury at the end of the 50–mile section of the race. There was an 18 percent higher rate of kidney injury among those who took the drug compared to those who didn’t, the study found.

“Basically, for every five runners who took ibuprofen, there was one additional case of acute kidney injury. That’s a pretty high rate,” he said.

This study should cause endurance athletes and distance runners pause before taking ibuprofen while competing, but does not infer that the average athlete would necessarily face similar effects from taking the drug, Lipman said.

“I would generalize to say, yes, caution should be warranted taking ibuprofen during long distance runs or other endurance sports events,” he said.

“I felt surprised and a little shocked that it really is as bad for you as we found,” said Lipman, who has now switched to using acetaminophen, such as Tylenol, for pain relief and taking ice baths after racing. “I feel it’s ironic to preach moderation in extreme sports, but moderation is probably a safe approach. If something hurts, these athletes might want to consider taking acetaminophen instead.”