To avoid the bias that can arise fromthe inclusion of prevalent users
(i.e. patients already taking simvastatin at the beginning of the study periodwhomight
carry a different risk of rhabdomyolysis, and be taking a
different dose, from those who started the drug during the study period)
[29], we restricted the study cohort to those patients who initiated
a new episode of simvastatin use during the study period. We cannot
entirely rule out the possibility that we inadvertently included some
prevalent users in the cohort (because of the incomplete NHI coverage
in 2005); however, the proportion would be extremely small and
should not have distorted our findings.