US National Library of Medicine: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0024264/
MedLine Plus: http://www.nlm.nih.gov/medlineplus/ency/article/007313.htm
National Institute on Drug Abuse: http://www.drugabuse.gov/publications/research-reports/heroin/how-does-…
http://www.drugabuse.gov/related-topics/trends-statistics/infographics/…