A blood taste or metallic taste in your mouth after exercise can be caused from a variety of things, although the precise cause -- unless it is tied to some underlying health condition -- may largely remain a mystery. From asthma to lactic acid, old fillings in your mouth to hemorrhagic pulmonary edema, there are many speculations as to what causes this. For the best diagnosis, consult your physician and be sure to give him a complete description as to when your symptoms occur, for how long, and what activity preceded them.
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The report of tasting blood in the mouth after exercising is often coupled with tasting it after an intense workout. Many elite athletes, runners, cyclists, and swimmers suffer from this mysterious problem. Complaints of a blood or metallic taste can often show up when exercising in cold weather when your mucous membranes can become irritated. If the intensity is what sparks this issue for you, begin to document your heart rate to notice any correlation on when this occurs.
Asthma, sinus problems, and excessively dry air may spur the taste of blood during exercise. If you are not asthmatic or do not suffer from sinus issues, it could also be caused when blood is forced through membranous tissue in the lungs, or even up into the bronchial tree. This occurs when you push yourself to the threshold of an anaerobic state, when the cardiopulmonary apparatus no longer benefits. Knowing this, it may explain why athletes notice this more than others. Although it is apparently not harmful, it may be best to limit the frequency of exercise of such high intensity.
Pulmonary Blood-Gas Barrier
Intense exercise in athletes can alter the structure of the pulmonary blood-gas barrier, causing mechanical stress and even blood leaking into the side regions of the lung, report Hopkins et al. in "Intense Exercise Impairs the Integrity of the Pulmonary Blood-Gas Barrier in Elite Athletes." In severe cases, some athletes have even coughed up blood.
Much of the research related to a blood taste in the mouth during exercise is related to elite endurance athletes, which doesn't explain why average runners or those occasionally involved in intense workouts may experience the same symptoms. To rule out any serious issues and properly determine the cause, consult your doctor. In the meantime, consider altering the intensity of your workouts so you don't push beyond your limits.