The purpose of this trial is to investigate a novel treatment for cannabis dependence: cannabidiol. Between 96 and 168 young people who want to quit cannabis and meet criteria for moderate cannabis use disorder (DSM-5) will be recruited from the community. Stage one aims to identify the Most Effective Dose (MEDmg) of oral cannabidiol for reducing cannabis use over four treatment weeks. Stage two will determine whether the MED identified in stage 1 can offer an effective treatment for cannabis dependence.

Inclusion Criteria:

• Aged between 16 and 60 years old.
• Meet DSM-5 criteria for moderate cannabis use disorder (≥4 DSM-5 criteria)
• Express desire to quit using cannabis within the next four weeks,
• Have ≥1 previous failed quit attempt.
• Smoke tobacco with cannabis,
• Test positive for recent cannabis use according to urine analysis,
• Vital signs within healthy limits and have capacity to give consent

Exclusion Criteria:

• Not willing to use effective contraception from when consent is taken to 6 weeks after treatment has stopped
• Positive pregnancy test or breastfeeding
• Allergies to the Investigational Medicinal Product (IMP) or placebo and its excipients
• >twice/month use of other illicit drugs
• Outside normal Body Mass Index (BMI)
• A physical health problem deemed clinically significant
• The use of current prescribed psychotropic drugs
• Current or prior self-reported diagnosis of a psychotic disorder
• Non-English speakers due to verbal assessments.