Ayahuasca is not the latest party drug but a foul-tasting plant concoction Amazonian people have been downing for centuries. It is the stuff of legends, credited with sending people on the most incredible trips. Today this bitter tea, also known as hoasca, has become the sacramental ritual of two modern religions in Brazil; one of them, the União do Vegetal (UDV) church, has invited McKenna, an expert on psychoactive plants, and other research teams, to scrutinise this sacred brew.