Alcohol and cannabis, alone or together, and whether people step in to stop a video

The goal of this randomized clinical trial is to learn how alcohol use, cannabis use, and using both substances affect whether and how quickly people stop a video being shown to a woman. The study will recruit 312 people from the Metro-Denver area who drink alcohol and use cannabis; half of the participants will be women. The main questions it aims to answer are:

• How do alcohol, cannabis, and their combined use affect whether participants stop the video and how quickly they do so?
• Does alcohol use versus alcohol and cannabis use by others in the scenario affect whether and how quickly participants stop the video?

Researchers will compare four randomly assigned groups: alcohol only, cannabis only, both alcohol and cannabis, and neither substance. Participants will also be randomly assigned to one of two scenarios involving alcohol use or alcohol and cannabis use by others. These comparisons will help researchers understand how participants' own substance use and substance use by others affect their decisions to intervene.

Participants will:

• Complete a phone call to review the study and schedule a mobile laboratory visit at their home.
• Have their ID checked, complete a breath alcohol test to confirm no alcohol is in their system, and be weighed. Female participants will also take a pregnancy test.
• Provide written consent and complete questionnaires so researchers can confirm eligibility before random assignment.
• Follow their assigned substance-use condition. Participants in the cannabis-only group will smoke their usual cannabis at home while staff monitor them through Zoom. Those in the alcohol-only group will drink two weight-based alcoholic beverages, with a target blood alcohol concentration of 0.08%. Those assigned to both substances will use cannabis first and then drink alcohol. Those assigned to neither substance will not use either as part of the study.
• Provide blood and breath samples to measure cannabis-related THC and alcohol levels.
• Complete a task in which they can choose to stop a video being shown to a woman. Researchers will measure whether they stop the video and how long they take to do so.
• Take part in a recorded interview about their experience and the reasons they did or did not stop the video. Researchers will analyze these interviews to better understand how substance use affects decisions.
• Answer questions about what they believed the study was testing and their impressions of the other participants, receive an explanation of the study, and receive compensation.