Why do depression or cannabis seem to help heroin-only users on buprenorphine to quit heroin?

See project summary here.

No crack use predicted heroin abstinence at follow up only when occurring with depression or cannabis use

Depression

Putative antidepressant properties of buprenorphine have been suggested in several other studies on mainly heroin-only users.

For the crack users in our study reporting depression, no beneficial effect of buprenorphine was observed. Cocaine use has been associated with depressive disorders, and cocaine dependence is strongly related to substance-induced depression. This suggests that cocaine use could both maintain and enhance depressive symptoms and is associated with a worse outcome. Thus for those suffering from depression but using crack cocaine, the antidepressant effects of buprenorphine might not be sufficient to balance additional mood disturbances induced by stimulant use.

Cannabis use

A positive effect of cannabis use may reflect a self-medication attempt; people shift their psychological attachment from heroin to cannabis as the drug providing immediate emotional relief.

For the crack-users in our study, no beneficial effect of cannabis on heroin use was observed. This could be due to the strong association between crack and heroin, which has already been proposed by others.