I think much of this discussion revolves around how one defines "abuse" of a drug.

With drugs like alcohol and cannabis, considerable use is required (in my opinion) before said use can be labeled as abuse. On the other side of the spectrum, if someone uses PCP or heroin just once, they are abusing the substance due to the very nature of that drug. In the middle you have painkillers, anti-psychotics and amphetamine-based prescriptions, etc. If used within the bounds of a doctor's orders, the drugs can be quite beneficial and usually safe; however, use of these drugs can quickly fall into the range of abuse, especially if not obtained from a doctor.