What factors actually affect psilocybin sensitivity? Weight, sex, metabolism, genetics

Body weight has a much smaller effect on psilocybin sensitivity than most people expect. In clinical trials, weight-adjusted dosing (mg/kg) has not consistently outperformed flat dose protocols. The reason: psilocybin's primary mechanism is receptor binding in the brain, and brain size/serotonin receptor density varies less with body weight than many assume. The practical implication: a 140-pound person and a 220-pound person may respond similarly to the same dose of psilocybin — more than they would to, say, alcohol, which distributes by body water.

Sex differences: some research and clinical observation suggests women may be more sensitive to psilocybin on average, possibly related to hormonal effects on serotonin receptor expression or metabolism differences. This is not well-established in controlled studies and individual variation within each sex is larger than the average between-sex difference. The practical advice is the same regardless: start lower, track your own responses, adjust accordingly.

What does reliably affect sensitivity: prior experience (tolerance develops quickly with repeated use within a short period), concurrent SSRI/SNRI use (significantly blunts response via 5-HT2A downregulation), mindset and expectation (a large portion of apparent 'sensitivity' may be expectation effects), and the specific metabolic enzyme CYP2D6 (involved in psilocin metabolism). CYP2D6 poor metabolizers may have longer and more intense psilocybin experiences. Genetic testing for CYP2D6 status is available through consumer genomics platforms.