The man, named as Henry Miller, 19, from Bristol, was in a remote rainforest area of Colombia with other tourists when he took the drug with a local tribe.
Mr Miller, who was due to go to university in September, is understood to have taken Yage, which brings on vivid hallucinations and supposedly spiritual experiences.
A fellow traveller said he drank a cup of the drug twice, on Sunday and Tuesday, while in the remote town of Mocoa in the Putumayo region.
Christopher Dearden, who is half French and half English, met Mr Miller on Sunday when they travelled together from a hostel in Mocoa to land belonging to a local shaman.
Mr Dearden said the drug had no effect on Mr Miller the first time but on the second occasion he became very ill.
He said the group of about eight people, including Mr Miller, were all sick after drinking the drug, a normal reaction to its unpleasant taste.
But while the rest of the group came around from the "trip", Mr Miller did not.
Mr Dearden, 27, said: "He just got worse and worse. He was lying face down on the ground making very weird breathing noises. We picked him up and put him in a chair.
"He wasn't speaking, he was lashing out with his hands and feet. Then he started making weird animal noises, pig sounds and at one point he tried to fly. He kept saying, 'What's going on, oh my God' and holding his face."
The shaman's family told the tourists they would look after him but when they woke up in the morning Mr Miller was not there.
Police arrived and showed them a picture of Mr Miller's body, which was reported to have been dumped by a dirt road.
Mr Dearden, who lives in France, said police told them he had a crack on his head and that they thought he may have fallen or been thrown from a motorbike.
He said: "Henry was a complete innocent. He had no ego, he was very kind. He was the last person this should have happened to."
A spokesman for the Foreign Office said: "We are aware of the death of a British national on April 23 in Colombia. We are providing consular assistance to the family at this very difficult time."
Yage, also known as Ayahuasca, is a psychedelic drink made from leaves and is used by native people in South America for healing and spiritual purposes. It is also known to cause nausea, diarrhoea and psychological distress.
The effects of the drug were documented by writer William S Burroughs in his book The Yage Letters, in which he wrote to poet Allen Ginsberg of his mind-altering experiences.