Hi. Welcome to the forum and thanks for the feedback.
Is your main issue that people overdose too frequently? I wouldn't be too surprised if that is the case as the game's balance is very "make do" at the moment. However that's an easy thing to sort in the next version if so.
I really do like the idea of drug users having less chance to overdose but having a shorter life span to compensate. That could be neat. I'll add it our long list of stuff to look at. I can't say when we'll actually get around to tweaking it but it's definitely something I'll strongly consider for a future version.
The main issue is that encouraging drug use leads to 0 drug addicts quickly. I would like to create a city where is almost everyone addicted and young because of a limited lifespan.
While we already have a citizen profile, maybe there is way register how long they use drugs and increase overdose accordingly. THe longer they use drugs the higher chance of overdose they have because they are catching the initial high they can never get again thanks to increasing resistance. Also, their bodies are more and more destroyed.
The formula can be something easy like this: base probability * habit duration (or the number of hits if we can store/retrieve that data easily).
Alternatively, this issue could be also gained from addicted parents or by working population(not sure of the architecture allows this).
PS: maybe in future current state can be also one of options. We could have eg. a chemical hut producing dirty drugs and resolve drug issue by mass suicide. Anyway, if we consider that the current drug is being used in hospitals i would consider it safer by default.