A recent review of a study done by Michael-John Milloy at the University of British Columbia in Vancouver (reference New Scientist, 3 June 2017) about using cannabis.
The group reviewed 3,000 people with a history of drug use. They asked these patients if they started to use cannabis to help them reduce their cravings for crack. Of this group, 122 crack users admitted to using marijuana to help them get off crack. They found that 28 percent of those questioned who knowingly used marijuana were off crack. This was a single, non-scientific study that suggests a role for cannabis in treating crack addicts.