The pharmacokinetic (PK) and clinical implications of combining metformin with rifampicin are relevant to increasing numbers of patients with diabetic tuberculosis (TB) across the world and are yet unclear. We assessed the impact of rifampicin on metformin PKs and its glucose-lowering effect in patients with diabetic TB by measuring plasma metformin and blood glucose during and after TB treatment. Rifampicin increased metformin exposure: plasma area under the plasma concentration-time curve from time point 0 to the end of the dosing interval (AUC0-τ ) and peak plasma concentration (Cmax ) geometric mean ratio (GMR; during vs. after TB treatment) were 1.28 (90% confidence interval (CI) 1.13-1.44) and 1.19 (90% CI 1.02-1.38; n = 22). The metformin glucose-lowering efficacy did not change (Δglucose - Cmax ; P = 0.890; n = 18). Thus, we conclude that additional glucose monitoring in this population is not warranted. Finally, 57% of patients on metformin and rifampicin, and 38% of patients on metformin alone experienced gastrointestinal adverse effects. Considering this observation, we advise patients to take metformin and rifampicin with food and preferably separated in time. Clinicians could consider metoclopramide if gastrointestinal adverse effects occur.
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