Is it a bad idea to wear thong underwear when I work out?
Honestly, it's not great. Thongs can create a lot of friction and irritate the delicate skin in your nether regions as you're running, biking or doing any other repetitive movement. This can mean extra discomfort if you've got hemorrhoids, which are swollen veins that can cause severe anal pain and itching (they're common post-childbirth).
Thongs also make it easier for bacteria to travel from your backside to your front, increasing your risk of urinary tract infections; this is especially a problem when you're sweating because the added moisture provides a perfect environment for bacterial growth. It doesn't help that these undies are often made from nonbreathable fabrics like spandex, which can trap moisture and make other issues, such as yeast infections, more likely.
This is not to say you should go commando whenever you're at the gym in less than ideal underpants—it's good to have some fabric between you and your running shorts. But the smartest advice is to pack a pair of full-back cotton underwear in your gym bag, or invest in shorts or workout pants with built-in moisture-absorbing panties.
Health's medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.