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The risks are very minimal – they don’t encourage bacterial growth or stop vaginal cleaning, and they don't absorb vaginal fluids so no additional risks overnight like with tampons, so the risks are far less then with a tampon, at the very most you may notice a little bit of a bad odour but that’s about it.
I normally use menstrual cups, I once forgot mine for several days, even had sex with it in before I noticed it wasn’t in the draw where I keep it!! No ill effects (and my vagina is a diva), there was a little odour for about an hour after removal, but that was all, and I’ve known a few women accidently leave them in too long - softcups and menstrual cups have similar health benefits.
Watch out for the normal signs of infection, just in case.