Oral contraceptives and the incidence of multiple sclerosis

Background: Experimental and clinical data suggest a protective effect of e strogens on the development and progression of MS. Methods: We assessed whe ther MS incidence was associated with oral contraceptive use or parity in t wo cohort; studies of U.S. women, the Nurses' Health Study (NHS; 121,700 wo men aged 30 to 55 years at baseline in 1976) and the Nurses' Health Study I I (NHS II; 116,671 women aged 25 to 42 years at baseline in 1989). Particip ants with a diagnosis of MS before baseline were excluded. Oral contracepti ve history and parity were assessed at baseline and updated biennially. Dur ing follow-ups of 18 years (NHS) and 8 years (NHS II) we documented a total of 315 definite or probable cases of MS. Results: Neither use of oral cont raceptives nor parity were significantly associated with the risk of MS. As compared with women who never used oral contraceptives, the age-adjusted r elative risk (95% CI) was 1.2 (0.9, 1.5) for past users, and 1.0 (0.6, 1.7) for current users. Similar results were obtained after adjustment for lati tude ancestry, and other potential confounding factors. There was no clear trend of MS risk with either increasing duration Of use or time elapsed sin ce last use. Age at first birth was also not associated with the risk: of M S. Conclusions: These prospective results do not support a lasting protecti ve effect of oral contraceptive use or pregnancy on the risk of MS. The dec ision to use hormonal contraception should not be affected by its effects o n the risk of MS.