Background: Exposure to first-line anti-tuberculosis (TB) drugs varies significantly between individuals and populations. We developed population pharmacokinetic (PopPK) models using observational data from persons receiving fixed-dose combination (FDC) to evaluate the association of HIV, DM, and toxicity with drug exposure. Methods: RePORT-Brazil participants with drug-susceptible, culture-confirmed pulmonary TB treated with rifampin, isoniazid, pyrazinamide, and ethambutol (FDC) were included. Samples were obtained at month 1 of TB treatment. Age, sex, weight, DM- (according to glycated hemoglobin, non-DM ≤5·7%, pre-DM 5·8–6·4%, and DM ≥6·5%), HIV-status, and NAT2 profiles were examined. Adverse drug reactions (ADRs) were graded for severity. Liquid chromatography/Mass Spectrometry quantified drug concentration. Time since last dose was self-reported; samples >8h post-dose and with undetectable levels were excluded. Monolix® was used for model building and individual parameter estimation. AUC0-8 exposures were compared by DM-, HIV-status, and ADRs. Findings: Of 388 unique samples (one per participant), 49 (13%) were excluded. Rifampin had the highest rate of undetectable levels (n=45, 13%), which was associated with shorter self-reported time between dose intake and sample collection (median 1·8 vs 3·0 hours, p<0·001) but not with other baseline characteristics. One-compartment PopPK models adequately described all drugs. AUC0-8 was lower among people with HIV (PWH) for rifampin (p=0·008) and ethambutol (p=0·022). Pre-DM and DM were associated with lower rifampin exposure (p=0·026 and p=0·004, respectively), and pre-DM with lower pyrazinamide exposure (p=0·027). PWH were more likely to experience grade ≥2 ADRs compared to HIV-negative individuals (30% vs 15%, p=0·003). Isoniazid exposure was higher among participants with grade ≥2 ADRs compared to those without (79·7 vs 73·5 mg·h/L, p=0·02). Interpretation: In people receiving TB treatment, four PopPK models demonstrated lower rifampin and ethambutol exposure among PWH, lower rifampin exposure with pre-DM and DM, lower pyrazinamide exposure with pre-DM, and higher isoniazid exposure among those with grade ≥2 ADRs. Funding: National Institutes of Health