Follow the shaman
Because of the interest from foreigners, Ayahuasca has exploded in Peru, and there are tons of touts in Iquitos trying to cash in. The stuff's suuuper potent, though, so you should really only even consider taking it with an experienced shaman, in a safe place.
There are several retreat centers throughout Peru that offer controlled Ayahuasca experiences -- do your research before arriving, rather than just accepting the first cup that comes your way. There've been stories of travelers drinking Ayahuasca from fakes and then being robbed or assaulted while they’re under. Not cool.