On: Ketamine no better than placebo at alleviating depression, unusual trial finds

[taraparo]

The Lumenate app.

Or if you don't count oxygen as a drug: Breathwork (Holotropic Breathwork, Quantum Light Breathing, Shamanic Breathing, Rebirthing, etc.).

Never tried but I heard that multi-day retreats in complete darkness can also induce some kind of (visual) trips.