Is DXM legal around you? Like Robittusin? If so, there are these guys it "should not be combined with":
isocarboxazid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, or tranylcypromine
If any of those are OTC as well, it might be a decent step 1 i think.