Ayahuasca is a drink made from two plants native to the Amazon, Banisteriopsis caapi and Psychotria viridis, which contain the psychoactive chemicals: harmala alkaloids and dimethyltryptamine (DMT). The drink has long been used by aboriginal populations for its putative spiritual and medicinal benefits in countries such as Brazil, Ecuador and Peru. In the 20th century, ayahuasca spread beyond its native habitat and has been incorporated into syncretistic practices that are being adopted by non-indigenous peoples in modern Western contexts. Ayahuasca's globalization in the past few decades has led to a number of legal cases which pit religious freedom against national drug control laws. Two principal constituents of ayahuasca are harmala alkaloids and DMT. These compounds, when ingested in combination, produce a unique biochemical synergy resulting in profound idiosyncratic psychoactive effects. Harmala alkaloids are controlled substances in some countries and DMT is prohibited by international drug control conventions. Therefore relatively little is known about therapeutic use of ayahuasca. This article provides an overview of ayahuasca and explores some of the medical and philosophical implications of contemporary ayahuasca use.