So I just encountered a corpse with completely destroyed internal everything, and the only external wound was a useless neck. He had critical internal poison, and for the first time ever I saw this while I was taking the poison:
You can not take any more internal poison.
So I flushed it completely and started transferring the rest. There wasn't much left, I got it all really quick and it was only "serious". However, after I got the finish transferring message and before I went to flush it, it went from serious to dangerous, and then to critical. I then flushed the snot out of it to get rid of the goofy stuff, and all went back to normal.
I suppose that must have been a greater-than-maxiumum potency poison...that or it was an automagically self-increasing poison. I can't think of any other reason I wouldn't be able to take all of it at once.
Thanks,
-Grand Master Empath Karthor