The researchers say that their results confirm those from previous studies – they do however provide further clarification of the VTE risk for individual drugs, particularly newer preparations (which previous studies have not had sufficient power to determine).
Current use of any combined oral contraceptive was associated with a significantly increased VTE risk (adjusted odds ratio 2.97, 95% CI 2.78 to 3.17) compared with no exposure in the last year. The estimated number of extra cases of VTE per 10,000 treated women per year (all ages) were higher for preparations containing desogestrel (14 extra cases; number needed to harm [NNH] over one year of 729), drospirenone (13 extra cases; NNH of 766), cyproterone (14 extra cases; NNH of 731) and gestodene (11 extra cases; NNH of 905). The risk was lower with preparations containing levonorgestrel (6 extra cases; NNH of 1739), norethisterone (7 extra cases; NNH of 1529) or norgestimate (6 extra cases; NNH of 1561).
There was no evidence that VTE risk varied according to duration of use, although there was a suggestion of a differential effect in women taking pills containing levonorgestrel. The comment notes that this finding is consistent with that of previous research, suggesting that new users of oral contraceptives are not, in general, at a materially increased risk of VTE compared with longer-term users.
The study has several limitations, mainly due to its design. These include for example the potential for misclassification of exposure, lack of information on women seeking contraception from other sources (NHS community contraceptive clinics), and a degree of uncertainty in VTE diagnoses (possible misclassification).
The authors believe that their results provide the most reliable estimates for VTE risk associated with COC use that are currently available from UK prescription data. They believe their study has the statistical power and sufficient adjustment for relevant confounders to be regarded as an important clarifying study. The authors of a related editorial say this research clarifies inconsistencies in earlier studies and provides important guidance for the safe prescribing of oral contraceptives.