Poison?
At one point someone puts something poisonous from under the sink into someone else's tea. I thought it was Michael and I thought it was into Rose's tea, but is there any confirmation of this in the movie? Other than that there's a cup of tea beside Rose's bed as she's going into convulsions? I find it difficult to imagine you wouldn't TASTE drain cleaner in your tea, but she is trippin' balls or whatever.
It also seems (from reading other posts) like there are about as many versions of this film floating around in the world as there are Michaels running around in the woods, so maybe it was explained more fully in some other edit.