4.2. Strengths and limitations
This study had several strengths and some limitations which warrant
discussion. The use of the Pharms database as a source of information
about drug exposure had several advantages. First, the database
contains records of subsidised dispensings of drugs for the entire population
of NewZealand.Moreover, it is highly likely that these records are
complete since pharmacists are not remunerated for such dispensings
unless they submit a claim. Second, the inclusion of an NHI with nearly
all simvastatin dispensings (increased from 91.5% in 2005 to 99.1% in
2009) meant we could identify almost all users of simvastatin during
the study period, obtain information about their use of other drugs,
and link to their hospital discharge andmortality data. Third, Pharms records
relate to drugs that were actually dispensed (not simply prescribed)
to patients. However it is still impossible to know whether
every patient took their medicines as directed, though the impact of
substantial non-adherence (particularly if it increased with increasing
doses of simvastatin because higher doses of simvastatinwere, in general,
less well tolerated) would be to underestimate the risks of rhabdomyolysis
and myopathy in users of higher doses of simvastatin.