I forget exactly what drug it was Karen and Sally bought, but there is no such thing as a non-addictive drug, street or pharmaceutical (aside from LSD).
Adderall, which is amphetamine, is abused by both students and a few years back, CS players. Increases energy, better concentration. Highly addictive for anyone who doesn't have ADHD etc.
Benzodiazepines, which is what I think Sally and Karen wanted, while Karen wanted a specific brand. Ranges from sleeping pills to anti-depressants. Very easy to develop a dependency on.
People who uses drugs to function in their daily lives will become addicted to them and we know Sally is struggling with her studies so she would be at serious risk.
I don't think this will be an actual plot point mind you, but Quinn is actively looking for customers and I'd say that's another strike against her.