Yage

Yage is a toxic plant that causes terrifying hallucinations. Users must save vs Will or suffer 5-20 minutes of horrors, with a 1% chance per minute of lasting insanity; those who save instead receive a vision of the future. Repeated use builds a cumulative 5% chance of becoming immune to its toxic effects, which is why some shamans and witch doctors favor it. It grows in jungle and costs 75 G.P. per dram.