Screening and Eligibility Assessment. Individuals will be screened by the research study intake coordinator. An initial pre-screen focused on inclusion/exclusion psychiatric diagnoses, medical status, current medication regimen, and ability and willingness to commit to completion of study procedures will be used to initially determine potential study eligibility. Interested individuals will be given a full description of the study procedures and asked to read and sign an IRB-approved informed consent form before participating in a detailed, comprehensive screening and assessment phase.

Diagnostic/Descriptive Assessment. The MINI International Neuropsychiatric Interview (MINI) will be used to assess psychiatric and substance use diagnoses. A medical history and physical exam will be conducted to ensure that the individual is eligible to participate. In the event an individual is found to be ineligible to participate in this research protocol, he or she will be given an appropriate referral for further medical care or to an appropriate treatment program. If found eligible, a randomization visit will be scheduled, and ongoing cannabis and tobacco use will be tracked between initial assessment and randomization.

Treatment Assignment. Eligible individuals will be randomized to receive varenicline or matching placebo in a 1:1 manner utilizing a stratified random block design with block sizes of 2, 4 and 6. To achieve treatment balance, randomization will be stratified by gender and presence of comorbid psychiatric conditions (depression, PTSD, etc.) as both have been shown to modify cannabis use. The randomization allocation schedule will be developed by the study statistician (Partially blinded, A/B treatment) and all study staff, the investigative team and participants will remain blinded to study assignment until study completion or an otherwise medical necessity. The study randomization will be reviewed by the study statistician (partially blinded) on a regular basis to ensure that the schedule is being implemented according to plan. Final treatment assignment (A/B) and dispensing will be performed by the VA Investigational Pharmacy, a centralized research pharmacy that manages clinical trial medications. Varenicline will be provided at the standard recommended dose of 0.5mg daily for three days, then 0.5mg twice daily for four days, and then 1mg twice daily for the remainder of the 12-week treatment period. Medication (varenicline and matching placebo capsules) will be compounded by Pitt Street Pharmacy, a local pharmacy that has been utilized previously by VA investigators. Dr. McRae-Clark holds an IND from the FDA for the use of varenicline in individuals with cannabis use disorder.

Medication Management. Medication Management (MM) is a common-sense, generalizable approach to encourage adherence to medication specifically, and to a treatment plan in general. Once a week, participants will be seen for a brief MM session. Sessions will be focused on (a) developing and maintaining rapport, (b) reviewing AEs and concomitant medications, and (c) discussing progress with medication adherence and cannabis reduction/abstinence. These sessions provide an opportunity to address challenges and help participants devise strategies for success. In some circumstances, when patient visits are not feasible, safe, or are prohibited, such as in the case of a pandemic, MM may be completed remotely by telehealth/phone. Remote sessions will be delivered via standard desk, laptop computer, tablet, or smartphone running VA approved applications, specifically VA Video Connect/Teams.