1. Drugs are frequently adulterated because of lack of any regulation. Adulterants are responsible for most of the side effects of heroin use. The only side effect of pure heroin is constipation.
2. Again, purity of drugs varies a lot, so there is no proper information available about dosages. This leads to overdoses.
3. Drug users are denied jobs and treatment, so they cannot get their lives on track even if they want to.
4. Money from drug sales is entirely in black, and is used to fuel further illegal activity.
5. Drug users don't have access to clean needles and safe paraphernalia, which leads to spread of blood-borne diseases and injuries.
All of these problems simply go away with legalization and regulation.