The Scientist: Rebecca Booth, M.D., is an obstetrician and gynecologist who practices in Louisville, Kentucky. She is the author of The Venus Week: Discover the Powerful Secret of Your Cycle…At Any Age.

The Answer: Toxic shock syndrome may seem like an urban legend. (Have you every known anyone who actually got it?) But make no mistake, T.S.S. is definitely real. It’s just really rare. There are between one and 17 cases per 100,000 people.T.S.S. has a number of possible causes, but it’s most infamous is keeping tampons in for too long. You should always change your tampon every four to six hours. That means you shouldn’t sleep with it in.

Your vagina’s pH is normally a very acidic 4.5, which keeps down the risk of vaginal infections. Blood’s pH, on the other hand, is a basic 7.4. The longer you keep in a tampon, the more time that menstrual blood stays in your vagina raising the pH. A higher pH makes the environment more hospitable to the Staphylococcus aureus (staph) bacterium that causes T.S.S. The tampon also allows in more oxygen than normal, which helps staph bacteria thrive.Many of us naturally have staph bacteria in our bodies, but they’re normally kept in check so they don’t replicate to dangerous levels. A soaked tampon produces perfect conditions for staph to multiply. This can lead to not only to T.S.S., which can give you a high fever, flu-like symptoms, nausea, a rash on your face, hands and trunk and possibly shock (hence the name), but also to yeast infections and smelly bacterial vaginosis.So consider using a pad at night and staying away from super-absorbency tampons, which can be painful to remove if they’re not saturated. The risk of T.S.S. is low, but maintaining balance down there is probably worth a couple extra trips to the ladies’ room.

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