this has been known among users of these drugs for quite a while now. They bond to the same receptors, so they interfere with each other. You can mitigate this a bit by going 24 hours without your SSRI/SNRI (ie skipping the dose that would precede the trip), and increasing dose of the psilocybin by ~1.5x. Or so I hear
Also pretty dangerous to do MDMA without pausing your SSRIs and just about deadly to do them when on MAOIs
Oh yes. Antidepressants have definitely reduced the effectiveness of various drugs for me.
Can confirm.