The effects of age, body mass index, smoking and general health on the risk of venous thromboembolism in users of combined oral contraceptives

Abstract

Objectives To investigate the factors associated with idiopathic venous thromboembolism in combined oral contraceptive users and to estimate the crude and age-specific incidence rates of idiopathic venous thromboembolism among this population.

Methods The UK MediPlus Database and the General Practice Research Database were searched to identify women with evidence of venous thromboembolism while exposed to combined oral contraceptives. Cohort and nested case-control studies were carried out using the same methodology on both databases. We conducted a meta-analysis using the individual data for the cases and controls from the two case-control studies to identify factors associated with idiopathic venous thromboembolism in women using combined oral contraceptives.

Results The incidence rate of idiopathic venous thromboembolism among oral contraceptive users was 39.4 per 100 000 exposed woman-years. The age-specific incidence rates were found to rise sharply after the age of 39 years. Factors identified as being significantly associated with idiopathic venous thromboembolism in women using combined oral contraceptives were: body mass index of 25 kg/m² and over, the association rising dramatically in women with a body mass index of 35 kg/m² or more; smoking; general ill health; and asthma.

Conclusion We believe that, before prescribing combined oral contraceptives, the venous as well as the arterial factors need to be considered and, in addition, age, obesity and smoking are all relevant when assessing an individual patient's risk.

KEY WORDS:

• Combined oral contraceptives
• Risk factors
• Incidence
• Case–control studies
• Cohort studies
• Meta-analysis