2009 Study: Occasional cannabis use improves treatment retention in treatment for opiate addiction.

From Society of Cannabis Clinicians
This study published in the American Journal of Addiction in 2009, replicates a previously surprising finding that intermittent cannabis use is associated with improved retention in naltrexone treatment among opioid dependent patients, while both abstinence from cannabis and regular cannabis use during naltrexone treatment are associated with high dropout.
http://cannabisclinicians.org/wp-content/uploads/2013/12/OpioidDependence2009.pdf