It's not that much of a derailment but as you wish.
But I will say that everyone knows the risks involved, Whether they can appreciate them or not isn't really relevant.
Maybe it's not, I don't know. I'll leave it to Skwim, I guess.
I'm not so sure that everyone
does know the risks involved. I'm quite surprised every day to realize that most people don't know the significant effects it has on the brain and body, never mind the psychological effects, environmental cues and genetic factors involved (some people are predisposed to addiction). Nobody tries cocaine for the first time thinking they're going to become physically and psychologically dependent on it, that they're going to re-wire the reward mechanisms in their brain to be satisfied only with cocaine and that they're going to end up broke and on the streets or possibly dead. And very few people just know how to quit once addicted to something, or where or how to get help, which is a real problem for someone addicted to heroine, cocaine or alcohol where suddenly abstaining from the drug results in very real medical and psychological problems and possibly even death. Have you ever seen someone with delirium tremens? People need real help to get off these drugs and its not always readily available.
So whether someone can appreciate the risks involved
is relevant, from where I stand. Not to mention all the people that become addicted to prescription medications (which is definitely beyond the scope of this thread) where choice doesn't play into it at all. So yeah, a person is making a conscious choice to try heroine, or alcohol or cocaine but nobody is making a choice to become an addict.
As to the relevance to the OP questions:
What could have been god's purpose in creating addictive drugs such as nicotine, alcohol, heroine, cocaine, etc?
And if they serve some reasonable purpose why make humans vulnerable to them?
I have no idea because I have no idea how a creator would want this for "his" creation. It's certainly not something I would build into the system, if I were a god.