The shamans who lead ayahuasca ceremonies are usually rather strict about who they'll let participate -- particularly if they're dealing with people from the first world, and the panoply of chemicals we put in our bodies. When I went to Peru to drink aya, we were specifically disallowed to take any Western medicines, except an anti-malarial, without consulting the shaman first (and had to discontinue use of them weeks before going down there), leaving completely aside the strict dietary and other restrictions during and for some time after the trip.

They've been doing this for thousands of years; I'm pretty sure they have far a better idea of the risks than your comment seems to me to intimate. (Assuming, of course, that they aren't just brujos, there primarily to take the gringo's money. That's not a problem with the medicine as much as it is with the practitioner, however. Even then, they likely know the risks very well; they simply ignore them. As with everything else, the doctrine of caveat emptor applies.)

Caveat emptor, indeed. If only there were some sort of rational method of inquiry involving painstaking scrutiny and freely published libraries of results were available to help consumers decide.