I have never encountered a colonist with chemical interest take drugs without being allowed to unless they are on a binge. In the tooltip it states "He/She will ignore directives to not use pleasurable drugs." This sentence suggests that the colonist will always take drugs whenever he/she feels like it. Is this misleading or have I been blind playing for the last 30 hours.
At least in A16, I had a Chemical Interest pawn who would crack open a beer to fill Joy during Joy time despite being set to "No Drugs." But when I used the Forbid tool to forbid all the beer, she avoided it until starting a binge.
Perhaps this has changed in A17, but if not, it does seem to work as the tooltip suggests: the pawn will ignore directives (meaning the drug use schedule) to avoid drugs. The wrinkle is that they WILL follow directives to not touch forbidden items, like a normal pawn, so you can relatively easily prevent them from using between binges. So perhaps all your drugs are forbidden, or perhaps they are just not located in convenient place when your pawn seeks joy activities.