https://wwwnc.cdc.gov/eid/article/14/9/07-0654_article. September 2008. Retrieved 2024-05-22. "crack cocaine users, other hard-drug users (predominantly heroin users but excluding those who used only alcohol and marijuana), and those not known to use drugs."
drugabuse.gov
"Commonly Used Drugs Charts". National Institute on Drug Abuse. National Institute on Drug Abuse. 2020-06-26. Retrieved 2020-06-24.