IntroductionTrimethoprim/sulfamethoxazole (TMP/SMX) is the antimicrobial of first choice in the treatment and prophylaxis ofPneumocystis jiroveciipneumonia (PCP) in immunocompromised patients, particularly in people living with human immunodeficiency virus (HIV). TMP/SMX use entails different adverse effects, and its association with early neutropenia is minimally documented. This study aimed to identify the risk of early neutropenia associated with TMP/SMX use in adults living with HIV in Mexico.MethodsA prospective cohort study was conducted in TMP/SMX-naïve adults living with HIV admitted to a third-level hospital between August 2019 and March 2020. Socio-demographic, clinical, and laboratory data were collected. According to patients’ diagnostic, if they required treatment or prophylaxis against PCP, medical staff decided to prescribe TMP/SMX, as it is the first-line treatment. The risk of TMP/SMX induced early neutropenia, as well as associated factors were analyzed through a bivariate model and a multivariate Poisson regression model. The strength of association was measured by incidence rate ratio (IRR) with 95% confidence interval.Results57 patients were enrolled in the study, of whom 40 patients were in the TMP/SMX treatment-group for treatment or prophylaxis of PCP (204.8 person-years of observation, median 26.5 days) and 17 patients were in the non-treatment group because they did not need the drug for treatment or prophylaxis of PCP (87.0 person-years of observation, median 21 days). The incidence rate of early neutropenia in the TMP/SMX-treatment group versus non-treatment group was 7.81 and 1.15 cases per 100 person-years, respectively. After adjusting for stage 3 of HIV infection and neutrophil count <1,500 cells/mm3at hospital admission, the current use of TMP/SMX was not associated with an increase in the incidence rate ratio of early neutropenia (adjusted IRR: 3.46; 95% CI: 0.25–47.55; p = 0.352).ConclusionsThe current use of TMP/SMX in Mexican adults living with HIV was not associated with an increase in the incidence rate ratio of early neutropenia.