Many people report that SSRIs neutralize the effects of mushrooms, making tripping only possible at much higher doses.
5-HT and psilocin both competitively bind to the 5-HT2A, so if you have more 5-HT at any given time, a greater percentage of the 5-HT2ARs will be bound to 5-HT, which means less will be available for psilocin to work it's magic.
Blessings
~ND
"There are many paths up the same mountain."