Depending on how susceptible you are, you could put yourself at risk for a bacterial infection -- not from your blood but from hand germs that got in with the cup. Your cup will probably start to smell bad and/or stain more stubbornly. And if it's a heavier flow day than anticipated, it could leak.

12ish hours isn't all together unusual for a light day for me (and I know there are others out there). You do run the heightened risks that kuradi8 mentions, but you're most likely going to be okay. My experience is that it'll smell bad, but with a good scrubbing it comes out. (The worst smell ever was killed off by a vinegar bath for one of my cups)

I left one in for 24 hours once. It smelled bad but that was it.

I got really sick at the end of my period once and forgot I had an Instead in that got left there for a couple of weeks - it didn't HURT me, but I don't recommend doing that. :)

Yes, you can safely leave it in for 12 hours. I've left mine in for longer - I work 12 hour shifts and put my cup in before I leave the house in the morning. I often don't have time to get to the bathroom - let alone empty my cup - at work (ah the joys of the nursing profession) so I don't usually get to take it out until I get home some 15-16 hours later. There was also one occasion when I was on vacation where I forgot I had my cup in for a full 24 hours. When I took it out it smelled bad and was a bit stained. It took boiling it in water with some vinegar to get rid of the smell and staining. I never experienced any ill effects (in fact I've never had any sort of vaginal infection in my life) though I wouldn't recommend making it a routine habit to leave it in that long.

I survived a left in tampon (super plus no less) for EIGHT days without so much as an infection. I'm sure not everyone's body would react the same way, but I at least feel confident if I forget my cup some time it shouldn't be the end of the world. That being said, in my whole life of using tampons and now cups, I've only ever left something in longer than 24 hours that one time.

How long is too long?? It seems to vary from person to person. I routinely leave my cup in for 24 hours at a time, and never had a problem at all. The manufacturers don't suggest doing this, but I suspect it is more of a liability problem of untested territory than really unsafe, IMO.

I'm a doctor, and I once saw a woman in the ER who left a tampon in for a whole month and only found it the following month when she went to put a new one in! She came to the ER pretty freaked out, as you can imagine, but she was physically fine - no infection, no problems. Now, I really do *not* recommend leaving the cup in for a whole month, but I'm sure it would be better than leaving a tampon in that long!

I've gone 48 hours with a cup in, possibly longer. I was using my Lunette continuously after I had my IUD inserted to help with the post-insertion spotting, and since it was so light and not the normal time to be having my period, I'd forget about it for days at a stretch and not empty/rinse it. I did not have any problems at all; not even bad smell issues, though that's not normally a problem for me even during my regular period, so YMMV. I regularly leave my cup in for 24 hours at a time during my period since even my normal flow is quite light.

The only thing I've noticed is that my cup gets smellier, but not irreversibly so.

I only empty mine every 24. I went 36 once, but it smelled pretty bad.

The one time I forgot it and left it in for 24 hours, it just had an awful smell. I just washed it out like usual and continued to use it and the smell faded with time. It was pretty quick, within the same cycle.

Sometimes it smells. A quick rinse gets rid of it for me. That's all I've experienced personally though, and towards the end of my period I actually leave it in for almost 24 hours once in a while.

It would increase risk of infections, TSS, along with bad odour and discoloured menstrual cup - however I'd still assume the risks are less than with tampons left for too long.

I've accidentally left a cup in for 5 days once, even had sex with it in without noticing it was still there. No infection or bad odour - and I'm very prone to infections, even using tampons, condoms or diaphragms correctly can trigger off infection for me.

What I find is odd discharge or blood in the cup - slightly bad smelling, and sort of a grainy look to the discharge or flow, with some of it sort of getting thick and sticking to the sides of the cup. I defiantly notice a difference to how the contents looks if it's been in too long, but nothing else noticeable.

Depends what brand you go with, some say 12 hours, some say 10, or 8... But just for the sake of keeping stains away, and because oddly, I actually like washing my cup, I will take it out every 4 to 8 hours, regardless of what day it is, and clean it with soap and water.

Also, as far as I know, cups don't really cause infections. But for some odd reason, if you've ever had an infection in the past, from that point on, even normal activities can cause it to come back. So sometimes its better to be extra careful with your cup in that case.

I remember a few months back one woman left it in for 2 months... she only discovered it when she was feeling herself down there during a shower. It smelled pretty horrible but she seemed fine otherwise. Not sure whether she posted in the community after that though.

I leave mine in for 24-48 hours routinely on my period. I actually forgot about mine for like 4 days straight this last period and I didn't die. :)