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I can't see any reason why the actives in milk thistle would be contraindicated when taken with psilocybin. This is the first I've heard of milk thistle being taken for depression, I thought it was more a liver supporting plant. Psilocybin itself may have anti-depressant properties...research looking at this is on-going at Imperial College and underway at Johns Hopkins.
Have you looked into other herbal anti-depressants? I think there is a lot more scientific evidence for the efficacy of turmeric over milk thistle, and for the treatment of a variety of other ills, depression aside. Its main active compounds curcumin and aromatic-turmerone have also been implicated in increasing neurogenesis, low levels of which are increasingly thought to play a role in depression and other cognitive disorders.
Some studies worth checking out:
Lopresti, A.L., Maes, M., Maker, G.L., Hood, S.D. & Drummond, P.D. (2014) Curcumin for the treatment of major depression: a randomised, double-blind, placebo controlled study.
Journal of Affective Disorders, 167, 368-375.
http://www.ncbi.nlm.nih.gov/pubmed/25046624 Sanmukhani, J., Satodia, V., Trivedi, J., Patel, T., Tiwari, D,, Panchal, B., Goel, A. & Tripathi, C.B. (2014). Efficacy and safety of curcumin in major depressive disorder: a randomized controlled trial.
Phytotherapy Research, 28,(4), 579-85.
http://www.ncbi.nlm.nih.gov/pubmed/23832433