I need a (non-slang) word or expression that means 'a harmful substance that is either controlled or strictly illegal, and that is abused (as opposed to used for medicinal purposes)'. I've thought about both drugs and narcotics, but since these can be used in both the abuse sense and the medicinal sense, they are not optimal. Question is – is there a non-slang word or expression like this in English? And if not, which of drug and narcotics respectively would you say would be the least ambiguous? I'm primarily asking natvie speakers of BrE – in AmE I guess narcotics would have more of the sense I'm after than drugs?
The best is probably "drugs" and allow context to clarify what you mean.
Drugs are strictly forbidden at this college.
If you need to specify, don't be afraid of adding an adjective: "Illegal drugs", harmful", "recreational" or "abused drugs", could all fit your needs.
There has been a fall in the amount of illegal drugs imported into the country.
A more technical term is "psychoactive drugs" or "psychoactives". These are drugs that alter mood, perception or behaviour, and include both legal (alcohol, nicotine) and illegal (LSD, heroin, cocaine). Nearly all drugs that are abused are psychoactive( but not anabolic steroids, for example).
Different cultures classify psychoactive drugs differently. So while "ethanol" is culturally accepted and commonly consumed in much of Europe, it is an illegal drug in parts of the Islamic world.
"Narcotic" is a rather poorly defined category, with different meanings to different people, and is probably best avoided if you are uncertain about its use.