Q. After I have a bowel movement I sometimes get quite weak and have to lie down until the feeling passes. Why does this happen and is it cause for concern?
A. The sensation you are describing is most likely due to a reflex called a "vasovagal reaction." Here's what I mean.
Often people need to tense their abdominal muscles and strain a bit during a bowel movement. This tends to stimulate the vagus nerve, which slows the heart rate.
At the same time, blood flow back to the heart decreases, so blood pressure drops. The combination of a slower heart rate and lower blood pressure can make you feel lightheaded and weak. I would expect the feeling to resolve after you lie down for a few minutes. But if it takes longer to recover, make an appointment with your doctor to make sure something else is not causing your weak spells.
— by Howard LeWine, M.D.
Editor in Chief, Harvard Men's Health Watch
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