LIMA - A Canadian man killed a British man after the two took a hallucinogenic plant brew known as ayahuasca together at a spiritual retreat in the Peruvian Amazon, authorities said Thursday.Witnesses told police the Canadian man, 29-year-old Joshua Andrew Freeman Stevens, killed the British man, Unais Gomes, 26, in self-defense after Gomes attacked him with a knife during an ayahuasca ceremony near the jungle city of Iquitos Wednesday night, said Normando Marques, a police chief in the region.Ayahuasca is a combination of an Amazonian vine and dimethyltryptamine (DMT) - containing plants that give users psychedelic experiences when combined. It is not normally associated with violence.The Canadian citizen was in police custody Thursday, Marques said.Witnesses said Gomes tried to stab Stevens during a bad trip, according to a police source in Iquitos familiar with the case.The source, who spoke on condition of anonymity, said Gomes apparently used a knife from the kitchen of the alternative health center Phoenix Ayahuasca to attack Stevens. Stevens ended up killing Gomes with the same knife, stabbing him in the chest and stomach, he said.Phoenix Ayahuasca did not immediately respond to requests for comment. Its Facebook page describes it as a safe place to "experience plant medicines and explore the true nature of the self."Ayahuasca tourism in Peru has surged in recent years, with dozens of jungle retreats offering the traditional indigenous brew to visitors under the supervision of a guide or shaman.Many tourists seek the drug out because of its reputation as a way to help ease depression and other mental troubles. Amazonian tribes in Peru and Brazil use ayahuasca, or yage, as an important spiritual and medicinal tool."It might be folkloric, spiritual or whatever else, but that doesn't mean it isn't a drug that dramatically alters your state of mind," said Marques.In 2012, an 18-year-old American man died during an ayahuasca retreat in the jungle and the shaman leading the ceremony buried his body in an attempt to cover up the death.