I personally use a non-drug supplement, from Smartpak, Smart Calm. It's just Magnesium and B1, both help with muscle relaxation. My mare has calmed significantly in just one month on it. She used to be so spooky, I counted 16 full spooks (where she gallops around rampant for several minutes) in 1 hour, just sitting unbothered in her field. Now that she's on this she can be out in very windy days and only spooks every once in a while or for only short little surprises. She's also much more focused when we work.
Magnesium and B1 seem to be the most common calming supplements, finding a good mix of those could be ideal, those are just dietary supplements, not drugs.