Introduction: Non-adherence to anti tuberculosis (anti-TB) medication greatly contributes to the rise in multi-drug resistance tuberculosis which is associated with high rates of mortality. Substance use is frequently seen among patients with poor adherence, however, little is known about the effect of substance use on adherence to anti-TB medications in Ethiopia. Therefore, we aimed to assess the effect of substance use disorders on adherence to anti-TB medications in Southwest Ethiopia.

Methods: A prospective cohort study was conducted among 268 patients with tuberculosis attending services at 26 health institutions in Southwest Ethiopia. A structured questionnaire was used to assess substance use disorders, adherence and other potential risk factors for non-adherence. Patients were followed for six months.  Generalized linear mixed model was used to identify the effect of substance use disorder on adherence to anti-TB medications. The model fitting was checked using Bayesian Information Criterion (BIC) while the covariate selection was based on a directed acyclic graph (DAG).

Results: The overall prevalence of non-adherence among patients with substance use disorders was 16.4% (n=22), 41.7 % (n=48), and 45.7% (n=59) at baseline, two month and six month respectively. The odds of non-adherence to anti-TB medications among patients with khat use disorder was nearly four times that of patients who did not use khat (aOR 3.8, 95%CI=1.8-8.0). Also patients who have alcohol use disorder were 3.2 times likely to have poor adherence compared to their counterparts (aOR=3.2, 95%CI=1.6-6.6

Conclusion: Khat and alcohol use disorders predict greater likelihood of non-adherence to anti-TB medication.