It varies dramatically, depending on the person, the type of medication, the type of psychedelic, and the dose.

My room mate smoked a full bowl of DMT on some SSRI, felt nothing. Yet shrooms worked for him.

My brother’s girlfriend also on SSRI’s felt nothing off psychedelics until one random time she dropped some acid with friends.

I know a guy who prefers acid when he’s on SSRI’s because it gives him “distance” so he’s not overwhelmed by what he witnesses.

SSRI’s also tend to block out or reduce MDMA’s effects.

This is an area where no one can give you clear answers - but it’s definitely not impossible for psychedelics to work with SSRI’s. The effects will almost always be reduced though, and often completely blocked. You’ll only be able to find out by experimenting.

Also: never take ayahuasca with SSRI’s, the harmala MAOI’s in ayahuasca can kill you when mixed with SSRI’s (or MDMA for that matter).

A suggestion if you REALLY want to trip: ask your doctor for moclobemide. Moclobemide is an MAOI antidepressant. It will actually make psychedelics in general significantly stronger. Never mix moclobemide with SSRI’s or MDMA, because as mentioned, MAOI drugs combined with SSRI’s or MDMA is a lethal and painful combo.

You can actual eat DMT on moclobemide... just like ayahuasca.