A meta-analysis of eight studies found that psychedelic drugs produce a rapid and sustained reduction in depressive symptoms, with effects appearing as early as day one and lasting up to six months. No serious adverse effects were reported, though transient increases in heart rate and blood pressure occurred compared to placebo. The analysis included studies published through April 2019.
A systematic review of 20 studies examined factors predicting response to psychedelic treatments for psychiatric and addictive disorders. The main predictor of a positive response across alcohol and tobacco use disorders, treatment-resistant depression, and anxiety and depressive symptoms in patients with life-threatening cancer was the intensity of the acute psychedelic experience. This factor did not predict response for obsessive-compulsive disorder. Possible mechanisms include modulation of the serotoninergic system via 5-HT2A receptor agonism, disruption and reintegration of the default mode network, or anti-inflammatory effects.
A systematic review of neuroimaging studies on psychedelics found that these substances reduce functional network integration, increase connectivity between brain networks, and expand the repertoire of functional connectivity patterns. Thalamic gating and emotional processing are also impaired. These changes correlate with symptom improvement in clinical populations. Despite these findings, understanding of psychedelic effects remains incomplete, and no unifying neurocognitive theory has emerged to explain the phenomena.