We have lots of rules to live by—some optional, some not. There’s the five-second rule (for picking up food that fell on the floor); the one-day rule (for texting that heart-eyes-emoji guy from the night before); and of course, the eight-hour rule (for changing your tampon).

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But if you forget to switch, or you’re stuck somewhere you don’t have a spare tampon, what’s the worst that can happen? We’ve all heard about the apparent price you pay for laziness—the terrifyingly named toxic shock syndrome—but we’ve also heard it’s super rare. So is it a real risk, or are we unnecessarily freaking out in the final hours?

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The long and short of it: Toxic shock syndrome (TSS) is extremely rare. It affects around one in 100,000 women, says Wendy Chang, M.D., partner and scientific director of Southern California Reproductive Center. As any tampon box will tell you, TSS is the result of an infection by two major kinds of bacteria, staphylococcus aureus (staph) and group A streptococcal (strep). Leaving a tampon in after the recommended eight-hour limit ups the risk, as does choosing the super-mega absorbency option (the more material, the higher the risk).

Turns out, everyone actually has some trace of staph bacteria on their skin, and in this case, in their vagina. That’s normally fine—it typically won’t do any harm. The trouble arises when that bacteria starts mass-producing, a situation that can be sparked by the extended presence of a tampon hanging out in your vaginal canal, says Sherry Ross, M.D., an OB/GYN at Providence Saint John’s Health Center in Santa Monica.

But that’s not to say leaving a tampon in for 12 hours spells certain death, or even an onset of TSS, Ross says. Lost or forgotten tampons are super common, sometimes for weeks at a time, and the only negative effect is a bad (OK, extremely foul) smell.

On the flip side, TSS can affect people at just nine hours—so what gives? Ross says it probably comes down to your staph bacteria at any given point. It’s still unclear what makes people more susceptible, but tampons give the bacteria a place to flourish—so the more material there is, the more room there is for them to grow (thus, the 80s jumbo-size scare). And when blood is present, it acts as a fuel—as Ross puts it, it’s “like a buffet for this staph to duplicate, because it’s so rich with everything it needs.”

Your Action Plan

Some rules exist for a reason. (That one-day texting thing? Not one of them.) Although TSS is super rare, gambling with your health is not a smart plan. And it’s safe to say we’d all rather make a trip to the bathroom (or drugstore) instead of the hospital.

That said, don’t freak out if you realize you’ve forgotten to change your tampon. If it’s only been left in for 12 to 24 hours, and you feel fine, just remove it and go on your way. Any longer than that or if you can’t remove it yourself, see a doctor for removal and a check-up.

In either case, you’ll want to keep an eye out for the flu-like symptoms of TSS, which usually include the sudden onset of fever, feeling sick, faint, and dizzy, says Jonathan Schaffir, M.D., an OB/GYN at the Ohio State University Wexner Medical Center. Another tell-tale sign is a rash that looks like a sunburn, sometimes all over—if you see that, get yourself medical help.