I've taken st. Johns wort with caffeine pills,
valium,
hydrocodone, alcohol, diet pills, and various combinations thereof and haven't seen any...never with SSRIs or antihistamines though, so I'm not sure...if memory serves the main problems with st. john's wort have to do with photosensitivity, so you'd probably be in more danger if you were taking it with artifiicial sweeteners than with piriton.