Dextromethorphan seems to fall into the same category- a friend of mine lost his son recently to an overdose of it. I'm not sure whether he was using it because it was more readily available than LSD, or because he preferred it, but he was a frequent LSD user and it seemed tragic that it was cough syrup that killed him.
I would worry more about the folks taking stuff like Ambien and Chantix, as those are legal and have well documented horror stories of people losing complete control.