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Overactivation of two receptors for histamine, normally associated with common allergies and acid reflux, may explain why some people, including highly trained athletes, pass out soon after heightened physical activities, according to researchers at the University of Oregon.

A series of studies led researchers in incremental steps to the discovery that the use of two commonly used antihistamines (fexofenadine and ranitidine) prior to exercise dramatically lower or completely eliminate low blood pressure following exertion. The drugs worked by preventing post-exercise hyperemia, an increased flow of blood, in the skeletal muscle during the critical 90-minute recovery period after exercise. In all, the pre-exercise consumption of the two antihistamines reduced the blood flow that occurs during recovery by 80 percent.

The study, funded by the American Heart Association, was posted online ahead of regular publication in the Journal of Applied Physiology. While fainting after exercise, a condition called syncope, can indicate a serious heart disorder, most cases are linked to low blood pressure and low blood flow to the brain.

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