To evaluate the effect size of type 2 diabetes mellitus (T2DM) on tuberculosis (TB) treatment outcomes and multi drug resistance (MDR).A cohort with 507 individuals with diagnosed TB included 183 with coexistence of T2DM and TB (TB-T2DM). Participants were identified at the time of TB diagnosis and followed during the course of TB treatment. Then we computed relative risks and adjustments by Cox proportional hazards for outcome variables (drug resistance, death, relapse, treatment failure), and the size of their effect as Cohen's-d.Patients with TB-T2DM were more likely to remain positive for acid-fast bacilli after two months of anti-TB treatment RR = [2.01 (95% CI: 1.3, 3.1)], to have drug resistant (DR) [OR 3.5 (95% CI: 1.8, 6.7)] and multi-drug resistant (MDR) TB [OR 3.5 (95% CI: 1.8, 7.1)]. The Cohen's-d for DR or MDR in T2DM was 0.69 when compared with non-DM subjects. The T2DM patients had higher odds of resistance to isoniazid (OR 3.9, 95% CI: 2.01, 7.9), rifampicin (OR 3.4, 95% CI: 1.6, 7.2) and pyrazinamide (OR 9.4, 95% CI: 2.8, 25.6), and their effect sizes were ≥0.67. Patients with TB-T2DM (versus no DM) were more likely to present with MDR TB (HR 3.1; 95% CI: 1.7, 5.8; p < 0.001), treatment failure (HR 2.04; 95% CI: 1.07, 3.8; p = 0.02) and relapse (HR 1.86; 95% CI: 1.09, 3.1; p = 0.02), with effect size ≥0.34.T2DM showed a substantial contribution to the presence of DR or MDR-TB and to adverse clinical outcomes during and after TB treatment. Our findings support the importance for routine screening of T2DM among newly-diagnosed TB patients in order to stratify them for immediate DR assessment, and highlight the need for clinical trials to evaluate variations to the standard TB treatment in TB-T2DM to prevent adverse treatment outcomes.