Oral contraceptive use has been associated with risk of Crohn's disease and ulcerative colitis.
Dr Hamen Khalili and colleagues from Massachusetts, USA determined whether this association is confounded or modified by other important lifestyle and reproductive factors.
A prospective cohort study was carried out of 117,375 US women enrolled since 1976 in the Nurses Health Study I, and 115,077 women enrolled since 1989 in the Nurses' Health Study II with no prior history of ulcerative colitis or Crohn's disease.
The researchers reported that these women had provided information every 2 years, on age at menarche, oral contraceptive use, parity, menopause status and other risk factors.
Diagnoses of Crohn's disease and ulcerative colitis were confirmed by review of medical records.
|The association between oral contraceptives and ulcerative colitis differed according to smoking history|
Among 232,452 women with over 5,030,196 person-years of follow-up, 315 cases of Crohn's disease and 392 cases of ulcerative colitis were recorded through 2007 in Nurses' Health Study II and 2008 in Nurses' Health Study I.
Compared with never users of oral contraceptives, the multivariate-adjusted hazard ratios for Crohn's disease were 2.8 among current users, and 1.4 among past users.
The researchers found that the association between oral contraceptives and ulcerative colitis differed according to smoking history.
Age at menarche, age at first birth and parity were not associated with risk of ulcerative colitis or Crohn's disease.
The researchers found that In 2 large prospective cohorts of US women, oral contraceptive use was associated with risk of Crohn's disease.
Dr Khalili's team concludes, "The association between oral contraceptive use and ulcerative colitis was limited to women with a history of smoking."