It’s made the rounds on runners’ forums for a while now, but the reason for that weird taste you get during a hard workout is surprisingly elusive. Ryan Dirks gets the question often in his role as physician assistant and director of team medicine at South Sound sports medicine clinic in Auburn, Washington.
"There's not always a simple answer—truthfully, we don't always know why it happens," he says. Further tests may be needed to figure out what's causing your specific symptoms, but there are a few simpler options.
First, Dirks says his team likes to rule out problems within the mouth itself. "In the same way you can get kidney stones, you can also develop stones in your salivary ducts, too, which can cause a blockage or sometimes an infection," he says. Physical activity and heavy breathing can increase saliva production, he adds, and may cause an infected duct to give off a foul, metallic taste. Tooth decay could also be to blame, for similar reasons.
If by "metallic" you mean "bloody," inflammation and tiny cracks in the lining of your nose and throat could also be to blame—especially if you're sucking in cold, dry air or if you're at higher altitude where the air is thinner. (Fun fact: Blood tastes like metal because of its iron content.)
Some studies also show that intense exercise can increase pressure on the lungs, which allows red blood cells to leak into air sacs, possibly causing that metallic taste. No cause for worry, says Gerald Zavorsky, PhD, associate professor of health and sport sciences at the University of Louisville. It’s only temporary.
Zavorsky's own research suggests another potential cause: A mild pulmonary edema, which is caused by when fluid leaks into the space between air sacs and capillaries. "This is what happens in most runners,” he says. In fact, Zavorsky published a study in June that showed mild pulmonary edema occurred in six out of the 36 marathon runners tested.
“It is so mild that there are no symptoms,” he says. But it could be another cause for the funky taste in your mouth, since the leaked fluid sometimes contains small traces of blood. Zavorsky notes that no studies have been done on this link specifically, and it's hard to say for sure.
Bottom line: If you’re in good health, practice good dental hygiene, and have no other concerning symptoms, the occasional odd taste during exercise is likely nothing to worry about. But it's worth mentioning to your doctor, especially if you've ever coughed up blood or had other respiratory or cardiac concerns.