I think Deathntaxes nailed the idea on the previous page.
I agree; trying to motivate the player into roleplaying "addiction" in a CRPG is difficult, if the player isn't interested in playing that out. However, presenting certain social opportunities where it would benefit the player to "party it up" to pass certain social skill checks / dialogues is a fantastic mechanic. Persuading the player into making questionable choices, rather than railroading them into decisions.
Oops apologies, I hadn't read the previous pages when I posted.
You're right, Deathntaxes' suggestion is excellent! I like the idea of a drug improving your social skills in the game and making the people you do it with like you more, with the caveat of having it affect you too much/for too long and making certain people like you less.
To expand on/repeat what you guys have said already: the drugs could make certain dialogue choices inaccessible and other accessible? And if you're addicted, the effects of the drug could last a very long time and restrict your behaviour more.
For example:
-You're addicted to synth coke because you chose to use it to gain an advantage too many times.
-You're talking to an attractive NPC, she could be really helpful to achieve something if she's cooperative.
-She offers you a line/pill/smoke.
-If you take it you will gain rep with her, but certain dialogue options won't be clickeable anymore (eg answers which involve modesty)
Since you're addicted, your one line turns into a bender, and you're still being influenced a while later.
-You encounter a character between whom there is a mutual dislike, but you need something from them.
-It would be better to approach things diplomatically.
-The drug, however, makes diplomatic options unavailable in response to taunts made by the NPC, so you can only click aggressive/egotistical responses.
-You end up in a fight, in a situation you otherwise wouldn't have wanted to be in one.