Rarity snoops through Sweetie Belle's bags and discovers a bottle of cough syrup. She admonishes her sister about it, then promptly ignores her own advice and downs the whole thing.
After all, what could go wrong?
Twilight accidentally ingests LSD
Rarity gets high on DXM
Fluttershy and Trixie shill drugs to doctors for money.
A new drug is released by Transpicuous.