Heroin

Heroin is apparently experiencing a slight increase in popularity thanks to its similarity to frequently abused prescription medications such as Oxycontin and Vicodin.  It's estimated that about 23% of people who use heroin will become dependent upon it.  To read about the many reasons to avoid using it (such as constipation, collapsed veins, and spontaneous abortion), as well as about treatment options, see this page from the National Institute on Drug Abuse: 

http://www.drugabuse.gov/publications/drugfacts/heroin

As with most drugs that people choose to abuse, heroin will certainly abuse you in return.