The risk of serious illness among oral contraceptive users: evidence from the RCGP's oral contraceptive study

Background. So far, no-one has attempted to evaluate the overall balance of serious, but not necessarily fatal, disease among a cohort of oral contraceptive users. Aim. To emprirically assess the balance of risk of serious illness among a cohort of oral contraceptive users followed up for up to 28 years. Methods. Oral contraceptive-associated serious disease was defined as that which is often life-threatening and/or associated with long-term disability, and which has been found, or postulated, to be associated with use of combined oral contraceptives. Data from the Royal College of General Practitioners' (RCGP) Oral Contraception Study were examined to determine the rate of such conditions during 335 181 woman-years of observation in 'ever users' and 228 727 woman-years in 'never users'. The rates were standardized for age, parity, social class, and smoking. Results. Compared with never users, ever users had a small increased risk of any serious disease (relative risk = 1.17; 95% confidence interval = 1.09-1.25). Ever users had an excess risk of cerebrovascular disease, pulmonary embolism, and venous thromboembolism, and reduced risk of ovarian and endometrial cancer. The increased risk was seen only in younger women; by the age of 50, ever users had the same risk as never users. The risk appeared to be confined to women using older oral contraceptives containing 50 micrograms or more of oestrogen. Conclusions. Past users of older, higher dose oral contraceptives can be reassured that the small increased risk of serious disease seen during current use does not persist after stopping, and that latent effects do not appear later in life. Currently available oral contraceptives, containing less than 50 micrograms of oestrogen accompanied by the progestogen, levonorgestrel, or norethisterone acetate, do not appear to be associated with an increased net risk of serious disease.