Table 1 Inclusion/exclusion criteria and their underlying rationale

Inclusion Criteria

Rationale

1) Informed consent

To ensure participants’ knowledge of what the study entails and to meet the requirements of the ethics committee

2) Minimal age of 18 years

To ensure that all participants are of legal age to consent on their own

3) Cannabis use at least once weekly over the last 30 days

To include participants not only with (almost) daily use and, thus, increase the study’s validity and generalizability

4) At least once weekly Internet access, including the potential to listen to audio(-visual) files while undisturbed

To ensure at least some access to the intervention and – in study arm 2 – to the mindfulness practices and video material

4) A valid email address and telephone number

A valid email address is needed for registration and to contact study participants (e.g., sending invitations to fill out questionnaires). A telephone number must be provided in case a participant cannot be reached via email (e.g., for follow-up surveys)

5) Fluency in the German language

To ensure that participants will be able to understand the information provided

Exclusion Criteria

Rationale

1) Participation in other psycho-social or pharmacological treatments for the reduction/cessation of cannabis use

To avoid confounding treatment effects

2) Current, pharmacologically-treated psychiatric disease or any history of psychosis, schizophrenia, bipolar type I disorder, or significant current suicidal thoughts

To avoid having subjects with these problems enter the study, and prevent confounding from other treatments