The killing took place on Wednesday in a village near the Amazon city of Iquitos during a shamanic ceremony for tourists that included drinking the hallucinogenic tea that is embraced by advocates as a spirit cure.

The supposed teachings of plants like ayahuasca, datura and tobacco attract travelers from all over the world. Some try to heal diseases, others try to find answers. In the past couple of years, dozens of retreats have been built near Iquitos to accommodate these practices.

Iboga and its chemical derivative, ibogaine, is not illegal in Costa Rica but the substance is also not regulated. The gray area occupied by the African root here has some drug officials concerned about possible health risks after Judicial Investigation Police have blamed at least one death on the drug.

"What I wasn't prepared for," Roland Griffiths says, "is people would come in two months later and I would say, 'Well, so what do you think of the experience?' And they'd say ... 'It was one of the most important experiences in my life.' "

About an hour had passed since I took the four-capsule microdose of dried iboga root, perhaps the most powerful visionary plant on Earth. Time seemed to have slowed down. I felt drunk, and my arms trailed beside me as I walked. Groovy.