If your tampon is filled with clear liquid it's probably been in for a while. It's OK to leave it in for up to eight hours, but after that, you should change it. "If you don't, it can lead to vaginitis, which is an inflammation of the lining of the vagina," Dr. Nathan explains. It can also cause toxic shock syndrome (TSS), which is a rare but serious illness that causes symptoms like fever and vomiting.
Leaving a tampon in for too long gives bacteria a place to grow. Your vagina is already full of good bacteria, but a tampon gives bad bacteria a cozy home. "The vagina is dark, warm and moist—all of which are bacteria's favorite environment," Youngblood says. "The tampon also provides an extra layer of protection, which can allow the bacteria to multiply more quickly."
Tampons also can leak, which may not be a big deal if you're on your period for a shorter time than usual and/or have a light flow. But if you're on a heavier flow or have a clotting problem, your tampon will fill up with menstrual blood and get wet.
That may lead to a wet string and painful leaks—especially if you're tucking it in too high, which can prevent the string from getting out of the way when you pee. To keep this from happening, always insert your tampon so that it's pointing up towards the back of your leg rather than down in front, and use a lubricant to help it slide in smoothly.