Background: We investigated the reasons for switching antiretroviral regimens, an issue rarely addressed in cohort studies. Methods: An observed toxicity switch rate (OTSR) was calculated by Poisson regression using the number of days individuals received each individual antiretroviral drug. Results: Of 3333 individuals receiving HAART, a total of 14% of regimens were switched, the majority occurring after 6 months of therapy. Toxicity was the major reason for switching (61%) and there were no major statistically significant differences in OTSR between the protease inhibitor (OTSR 26.4, 95% CI 18.3-37) and non-nucleoside reverse transcriptase inhibitor (OTSR 22.2, 95% CI 13.6-34.4) based regimes. For individual antiretrovirals, stavudine and zidovudine had significantly higher " switch" scores than all other drugs. Conclusions: There were no differences between the major HAART classes in OTSR. We suggest that newer antiretrovirals will require differentiation in terms of longer-term toxicity, as this is the major reason for switching.
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