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Safety aspects
The incidence of VTE in the remaining ‘healthy user’ group
then becomes less and less over time. This is the major factor
responsible for the ‘healthy user’ effect.
OC1
OC2
Number of adverse events
time
Fig. 16: User cohort effect (the ‘healthy user’ effect)
or attrition of the susceptibles
(Lewis et al 1997)
Selective
prescribing
A second biasing factor was also operating in the
epidemiological studies called prescribing bias or selective
prescribing. The third-generation Pills were selectively
prescribed to women with risk factors for cardiovascular
disease, such as age, personal or family history of VTE, clinical
venous signs, chronic inflammatory disease, anesthesia or
plaster cast, obesity, diabetes, standing working position,
alcohol abuse, smoking or a combination of risk factors
(Lunsen van 1996; Lis et al 1993; Poulter et al 1996;
Heinemann et al 1996; Farmer 1996; Herings et al 1996;
Jamin and De Mouzon 1996; Lewis et al 1996; Farmer et al
1997; Lidegaard 1997; Dunn et al 1998). As a result of
selective prescribing, cardiovascular adverse effects could be
more frequently observed in users of the third-generation OCs.
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