Dosage adjustments are often used to manage HIV drug interactions, but little is known about their clinical significance. We examined patients from the Ontario HIV Cohort Study to assess the effects of dosage adjustments on plasma viral load. A significant reduction (0.67 log10 copies/mL) in viral load was associated with adjustments to manage efavirenz-based interactions (95% confidence interval, -1.33 to -0.01) but was not observed after adjustments to manage rifabutin-based (difference in viral load, 0.03 log10 copies/mL; 95% confidence interval, -0.71 to 0.77) or nevirapine-based interactions (difference in viral load, 0.09 log10 copies/mL; 95% confidence interval, -0.83 to 1.01).