There are medications that if not used correctly harm everyone.

Antibiotics cause antibiotic resistant strains, and if you don't take the full dose it gets worse. As such their used needs to be monitored closely.

Some drugs cause mental issues. you shouldn't drive after taking alcohol - also don't drive after taking many pain killers. Again we need to watch this closely since you can harm someone else.

If someone I don't know harms themselves I could say none of my business (this is a debatable moral issue, but at least you can understand that side). However if you harm someone else that is a problem.