EFFECTS OF CHANGES IN SMOKING STATUS ON RISK ESTIMATES FOR MYOCARDIAL-INFARCTION AMONG WOMEN RECRUITED FOR THE ROYAL-COLLEGE OF GENERAL-PRACTITIONERS ORAL CONTRACEPTION STUDY IN THE UK

Study objective-To determine whether changes in smoking status among w omen recruited for the Royal College of General Practitioners' Oral Co ntraception Study affected previous risk estimates for myocardial infa rction. Design-(1) Postal survey between November 1994 and July 1995 o f women still under general practitioner observation. Validation of th e smoking information supplied by the women on the questionnaire by co mparison with that reported by the general practitioner at recruitment to the main study. (2) Nested case-control study of 103 cases of myoc ardial infarction, matched with 309 controls, to see if different risk estimates were obtained when smoking status at recruitment or smoking status at time of event were used in the analysis. Setting-650 genera l practices throughout the United Kingdom. Participants-10 073 women w ho responded to the questionnaire (85.4% of 11 797 sent out). Main res ults-There was good agreement between smoking information recorded by the general practitioner at recruitment and that supplied retrospectiv ely by respondents to the questionnaire. The risk estimates for myocar dial infarction associated with use of combined oral contraceptives (C OCs) were almost identical irrespective of whether smoking status at r ecruitment or at time of event was used for the statistical adjustment . This was because few women stopped smoking while also using COCs. In fact, fewer regular smokers who have ever used COCs reported stopping smoking than never users. The risk estimates for myocardial infarctio n associated with smoking were smaller when smoking habits at recruitm ent was used than when smoking habits at time of event was used. Concl usions-Previous results from the Oral Contraception Study regarding th e effects of COCs are unlikely to hare been biased by changes in the s moking habits of the cohort, but the effects of smoking have probably been underestimated. There is still a need for effective health educat ion regarding the risks associated with smoking, particularly among us ers of COCs.