Participants randomized to the intervention condition will receive the adapted Friendship Bench intervention.

Participants in the control group will receive usual care for their common mental disorders.

The Friendship Bench model comprises 6 weekly, individual sessions with trained peer workers in addition to the usual care. Peer workers will help participants identify problems, outline potential solutions, and implement selected solutions.

The adaptation of Friendship Bench for Vietnamese methadone patients made revisions, such as removing the 'Belief in supernatural powers' section, replacing the Shona Symptom Questionnaire with DASS-21, and adding meth use. But this adaptation removed women-focused content on pregnancy and postpartum depression that we plan to include.

The ability to enroll WWUM is assessed through the recruitment rate (number of WWUM approached to accrue the final sample) and reasons for non-participation.

Based on data from previous studies in Vietnam, the study team defines feasible retention as 80% retention at 6-weeks.

As peer outreach workers will conduct home visits or visits to workplace, the study team considers 70% participants attending all 6 individual sessions to be feasible. The reasons for session non-attendance, and counselor time per session and per patient including preparation, documentation, and supervision will also be assessed to explain the Friendship Bench completion rate.

Acceptability is defined as the perception among implementation stakeholders that the intervention is agreeable or satisfactory. The study team will assess acceptability through the 6-week follow-up interview with all participants (n=100) and peer workers (n=10). Acceptability will also be assessed qualitatively using the post-intervention in-depth interviews with peer workers (n=10), participants (n=20) and brief exit interviews with supervisors (n=2). The interviews will assess how easy the intervention is to participate and to deliver, the perceived usefulness of the intervention and suggestions for improvement. Intervention acceptability is defined as 80% participants rating the intervention as acceptable and by a common theme of positive perceptions of Friendship Bench in qualitative data.

The mental health outcome is defined to be common mental disorders symptom response and measured as a composite score. Common mental disorders symptom response is based on common mental disorders symptom improvement. Common mental disorders symptom improvement is defined as the percent improvement in each depression, anxiety and stress subscale of the 21-item Depression, Anxiety and Stress Scale (DASS-21) from baseline, averaged across only those subscales that were elevated at baseline. Each subscale ranges from 0 to 42, with higher scores indicating worse outcomes.