Drugs have side effects, so put down a minor chance to lose HP and a major chance of gaining HP, like 90/10 or something, and it would either give/lose less or more HP. Like !takedrugs then I gain 10 hp the first time, 13hp the second time, 8hp the third time, then I lose 5hp at the fourth time, etc.
Also, if it's going to be based in HP loss/gain only, why there's just 'Drugs' and 'Food' so you buy them, use them and RP whatever is the drug/food you are consuming? People will also just buy the food that will heal more and leave the ones that heal less behind.
Like !takedrugs then /me smokes some weed or !takedrugs then /me swallows a LSD pill.