A while back (some years perhaps) on Reddit there was a story about someone who basically documented their experiment with heroin. It could have been a hoax of course, but most agreed it wasn't.

It was this person who was already doing milder illegal drugs. He goes to his dealer and dealer proposes something different this time. He decided to try heroin. Had pretty high confidence he wouldn't get addicted. Anyway he describes his first try. Then as weeks went on he periodically posted updated. He kept justifying going back to trying it. Gets addicted. Long story loses his job, girlfriend, goes into rehab for a long time. Pretty harrowing story.

EDIT:

Thanks to japhyr56 who found the link:

http://www.reddit.com/user/SpontaneousH/submitted/