I'm pretty sure they can actually, at least in the US, as ridiculous as it sounds. There was a combination of dextromethorphan and quinidine (two very old and generic drugs) that was being advertised on TV in the US not that long ago (only non-generic drugs being advertised, for obvious reasons). I think it had been originally investigated as an antidepressant, but only got licensing for a somewhat obscure condition (pseudobulbar affect). The quinidine is just there to inhibit the metabolism into DXO, like cimetidine or the quinine in tonic water would.Remember they can't combine bupe with an existing generic drug like cetirizine because then they cannot patent the combination preparation. In order to secure a patent they need to combine bupe with a new drug. That's the one and only reason the samidorphan is there instead of them just doing clinical trials on buprenorphine alone.