If you are on them, you can still get positive results, but your baseline for microdose might be higher. Be careful if try… go slow, follow proper harm reduction measures.
I have a recent experience with MDMA, a substance that also interacts with those receptors. A friend and I both tried MDMA, not first realizing that one of us felt almost no effect because of an ongoing Lexapro prescription. We were well supervised in this endeavor, but I can imagine a more unrestricted experience in which you take way more than a recreational dose by accident.
By the way, you might need those SSRIs!