I would never do meth on purpose, but I am pretty sure some of the pills and coke I have done were cut with it. I take diphenhydramine to hasten sleep and potentiate opioids, but I will not take it in sufficient quantities to cause delerium. No datura either. No inhalants other than nitrous, nitrites, and ether. No needles unless administered by a medical professional. (Exception: I might consider IM ketamine. Maybe.) I have done lots of DXM, but I do not see myself doing it again, other than possibly to potentiate opioids.
Cyanide is probably a no-go as well... other than that, bring them on!