St. John's worth contains Adhyperforin - a gaba reuptake inhibitor. Phenibut is a GABA-B agonist so this could be comparable in taking an SSRI with MDMA which may dull the effects. I am not quite sure if the two drugs are safe to combine so I will suggest that if you are using this to treat depression and anxiety you speak to a Physician to make sure that you will not encounter any problems or adverse effects.
Having said this, I think it is best to move this thread to Basic Drug Discussion and I hope you find more answers to your question.