you may look at a multi-centre trial with family therapy for adolescent cannabis users (Schaub et al BMC Psychiatry 2014 14-26)

many thanks Ambros, particularly interesting that the coerced & self referred equally well, best wishes Ian

You may also see this interesting meta-analysis:

 also Davis, M. L., Powers, M. B., Handelsman, P., Medina, J. L., Zvolensky, M., & Smits, J. A. (2014). Behavioral Therapies for Treatment-Seeking Cannabis Users A Meta-Analysis of Randomized Controlled Trials. Evaluation & the health professions, 0163278714529970.

many thanks Marie this is really helpful, best wishes Ian

the following may also be of interest

Pacek & Vandrey in Experimental and clinical pharmacology (2014) 22:517-523

thanks Ambros I hadn't seen this paper and reading through it I can see it was exactly what I was looking for, so many thanks Ian