Girls who wear thongs may have the potential for bacteria from the rectal area to be dragged up towards the vagina or the urethra, increasing the chance of infection. Bare skin doesn't cause VPLs (visible panty line) either, so going commando can be a good alternative to wearing a tight thong.
The myth really is true: urinate before and after sex. It's very, very helpful. Doing that naturally flushes the bacteria from the urethra. That way, when you're thrusting, there isn't bacteria to colonize in your urethra.
Thongs aren't any worse than full-coverage underwear as far as promoting or decreasing the chance of infection or irritation—as with other styles, it's ultimately the materials that matter. Still, if you opt for a thong, go for one that fits properly and has a cotton crotch panel, as a too-tight thong can lead to chafing, particularly if it's a G-string style.
Cotton underwear, going commando or thong underwear with a cotton crotch are probably your best bets. Often, if someone is prone to infection, I'll tell them to sleep without underwear to aerate the area. I'll also tell them to put a hair dryer on cool when they come out of the shower and blow-dry their bottom to get rid of excess moisture.
Plenty of people have been wearing underwear to bed their whole lives and have never had a problem with itching or inflammation. If that's you, there's really no health-related reason to change your habits.
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