The demon drug is a mixture of honey, bitter bugs, and might seed. In the game's own words " boosts your attack by filling you with - guess what?- demonic strength". My theory is that when using demon drug you actually get possessed by said demon. My theory is based off of the hunter art in monster hunter x called beast mode ravenous wolf witch makes your hunter get enveloped in a black aura along with red eyes which is very possessed looking (bad English).