Psilocybin and pregnancy/reproductive health — what does the evidence say?

The honest answer is: the human evidence is genuinely very limited because pregnant women are excluded from clinical trials. What exists: (1) Animal teratology studies: psilocybin has been studied in rodents; no consistent teratogenic effects found at doses comparable to human therapeutic use. However, rodent data doesn't reliably predict human outcomes. (2) No human studies on pregnancy exposure to psilocybin specifically. (3) Population data on psychedelic use during pregnancy: some retrospective data from earlier eras when psychedelic use was more common, with mixed findings. The precautionary principle is reasonable given this evidence gap.

Regarding fertility and reproductive health: no specific evidence of psilocybin effects on fertility in either direction. Psilocin is rapidly metabolized (half-life approximately 1–2 hours); occasional use at significant intervals would mean no psilocin present at conception in any detectable amount. This is different from, say, cannabis (lipophilic, stored in tissue for weeks) or alcohol. Reasonable precaution: avoid psilocybin use if trying to conceive or pregnant.

The breastfeeding question: psilocin is water-soluble and would be expected to appear in breast milk at low levels shortly after use, then clear relatively quickly (given the 1-2 hour half-life). Timing breastfeeding to avoid the acute exposure window (pump and discard for 8–12 hours) is the approach some harm reduction resources recommend if a breastfeeding person does use psilocybin. However, the actual infant exposure and risk has not been studied. Abstention during breastfeeding is the precautionary default.