Examine pharmacology.

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As I'm sure you know, we cannot write the paper for you. We are not a term paper service. We only push you in the right direction with sources, structure, ideas, concepts, arguments etc.
I am doing cocaine, since you do not specify which you want.
Here are two great articles to start:
From NIH (http://www.nida.nih.gov/Infofacts/cocaine.html) and
http://www.nida.nih.gov/ResearchReports/Cocaine/Cocaine.html (this one has multiple parts)

This is a general bank of articles: http://www.nlm.nih.gov/medlineplus/cocaine.html

So let's see how a few ideas can develop. I know this is a personal "reflection," but, as with all of these, you need lots of facts, not just vague impressions to do well.
A few issues:
1. Long term vs short term effects.
This one should be hit hard. It is fairly well known that cocaine is addictive, but the reasons how and why are not. The issue is brain adaptability. That is, your brain gets used to the increase dopamine in the brain. This is all about motivation, thought processes and excitement for what you're doing (even if it's not all that exciting).
This is the real issue - once someone sees how high they can get, regular life is just boring. You're unmotivated. Your life does not matter. On cocaine (or Adderall, since these work in a similar way, by increasing dopamine levels), everything matters and you view yourself as an important part of society.
So, the person who gets addicted is one that loves the motivation and intellectual stimulation (think of writers and artists, musicians, etc).
The long term effects are all about the results of constant stimulation ...