A British man has been stabbed to death during a hallucinogenic ceremony in the Peruvian Amazon.

A Canadian man is in custody after killing the Briton, Unais Gomes, 26, in apparent self-defense on Wednesday night.

Witnesses said Gomes attacked the Canadian, 29-year-old Joshua Andrew Freeman Stevens, with a knife after suffering a bad trip.

The pair had been drinking a hallucinogenic plant brew known as ayahuasca at a spiritual retreat near the jungle city of Iquitos.

Ayahuasca, which is a combination of an Amazonian vine and dimethyltryptamine (DMT), gives users a psychedelic experience although it is not normally associated with violence.

"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," police chief Normando Marques said.

A local police source, who spoke on condition of anonymity, said Gomes is believed to have used a knife from the kitchen of a nearby alternative health center to attack Stevens.

Stevens allegedly used the same knife to kill Gomes, stabbing him in the chest and stomach, the source said.

Ayahuasca, also known as yage, is used by Amazonian tribes in Peru and Brazil as a spiritual or medicinal tool.

Dozens of jungle retreats offer the brew to tourists.

In 2012 an American teen died after drinking ayahuasca during a ritual in the Madre de Dios jungle region of Peru.

A shaman later admitted trying to cover up the death by burying him in the grounds of the retreat.

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