*Spoilers* (Something no one here seems to notice)
1) I agree with the guy in another post about the muffins, that is what I was thinking the whole time. They mention them 3 separate times during the movie so It's kind of obvious.
2) I believe, and this is just a guess of course, but The part where they all start popping pills to me felt like the cult leader was being cornered and he paged all of his members to take the pills. The reason they exchange pills instead of taking themselves is pretty obvious. I'm not a religious man but in most religions that I know of suicide is a sin, and this is further proved when the old lady says "God forgive me" when she takes her pill herself.
3)I also agree with the other poster about Karen's drugs wearing off and seeing nothing but corpses.
4)The beginning of the movie is actually the end, long in the future. Karen has a scar on her left shoulder, from that night. She also has nightmares of subways and creepy people on the train.
5)And finally the nail in the coffin to skeptics. The envelope in her dream during the beginning of the movie says: "To Karen Clavicep Purpurea Ergot Viviane" Claviceps purpurea, cause of ergotism and likely contributor to the Salem Witch Trials. Ergotism is caused by the chemicals in the fungus called ergot (pronounced AIR-got). Consumption of foods contaminated with ergot and ergot derivatives may cause vomiting, diarrhea, hallucinations, and may lead to gangrene in serious cases.(taken from http://botit.botany.wisc.edu/toms_fungi/oct99.html)
I believe it's signed Viviane because of the guilt she felt for letting her go before she was ready (Viviane was the asian girl who killed herself, likely from eating the muffins in the hospital)
Lastly also taken from (http://botit.botany.wisc.edu/toms_fungi/oct99.html) in case you are afraid or too lazy to click a link.
So what does this all have to do with the Salem Witch Trials, which took place near Salem Massachusetts in the late 1600's? There have been various attempts to explain those witch trials. None of them are more logical and interesting than the hypothesis of ergot poisoning, caused by Claviceps purpurea. The behavior was not identified as witchcraft until 1691, and this was just the beginning of the problem. Many people were sent to trial and often convicted and imprisoned. By September 1692 twenty men and women had been put to death for their crimes. All of the accused had similar symptoms: manic melancholia, psychosis, delirium, crawling sensations of the skin, vertigo, headaches, vomiting and diarrhea. All of these are symptoms of ergot poisoning, and it is likely that at list the initial hysteria was started by Claviceps purpurea infecting the grains of rye. This was chronicled in an article (Science 192:21-26, 1976) by Linnda R. Caporael called "Ergotism: The Satan Loosed in Salem?" She provides compelling, although circumstantial, evidence that the Salem witch trials coincided with a weather period that would have produced large quantities of ergot on rye, which was grown in the lowlands in that area.