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Objective. The purpose of this study is to determine the predictive factors of the isoniazid acetylator phenotype and its clinical impact in patients treated with first-line antituberculosis drugs. Methods. A retrospective study (2010–2020) conducted in the infectious diseases department in collaboration with the clinical pharmacology department, including all patients hospitalized for active tuberculosis who underwent an acetylation test after starting first-line antituberculosis chemotherapy. A descriptive, inferential, and predictive statistical analysis of data collected from medical records was performed. Results. Seventy-nine patients were enrolled in the study: 18 men and 61 women, with a mean age of 41.79 years (range: 13–83 years). Diabetes was the most prevalent comorbidity (10.1%). Lymph node involvement was the most common site of tuberculous (55.7%). All patients received first-line antituberculosis drugs. There were 58 slow acetylators (73.4%). There was no significant difference between the administered and recommended daily doses of isoniazid (4.79 mg/kg versus 2.89 mg/kg; r = 0.007; p = 0.955). Adverse effects were observed in 52 cases (65.8%). The outcome was favorable in 93.7% of cases. The multivariate analysis revealed no independent predictors of the acetylator phenotype. The isoniazid acetylation profile was not statistically associated with the occurrence of adverse effects or treatment failure. Conclusion. Pharmacological therapeutic monitoring of isoniazid is a cornerstone of tuberculosis management. Our study suggests that the isoniazid acetylator phenotype has no clinical impact in patients treated with first-line antituberculosis drugs.