Taking the oral contraceptive can cut women's short-term risk of multiple sclerosis, a study suggests.
Contraceptives contain hormones
The Harvard School of Public Health research showed the incidence of MS was 40% lower in those taking the Pill compared to those who were not.
The findings are in line with previous animal studies which suggested female hormones could delay the onset of MS, or the development of symptoms.
The three year study is published in Archives of Neurology.
MS is an inflammatory disease which causes a range of symptoms from fatigue and numbness to difficulties with movement, speech and memory.
Researchers compared 106 women who had a new diagnosis of MS between January 1993 and December 2000 with 1,001 other women without MS. Information was taken from the British General Practice Research Database.
Incidence of MS was 40% lower in women using oral contraceptives than in those who were not taking the Pill
Women were also found to have a lower risk of MS during pregnancy, but a higher risk in the six months after having a baby - compared to those who were not pregnant.
Writing in Archives of Neurology, the researchers led by Dr Alvaro Alonso, said: "These findings are consistent with studies on the effect of pregnancy in patients with MS and the immunological changes associated with pregnancy.
"Our findings suggest that high levels of exogenous [from outside the body] oestrogens from OC use and of endogenous [from the body] oestrogens during pregnancy may delay the first clinical attack of MS."
Chris Jones, chief executive of the UK's MS Trust said: "Any new information which highlights potential protective factors for MS is welcomed.
"It is already well documented that sex hormones such as oestrogen can influence the development and course of MS as evidenced by the higher ratio of women with MS compared to men, and the higher risk of MS in women post-pregnancy.
"But it is however difficult to draw firm conclusions from this one study.
"As the authors comment themselves, other factors may have influenced the results, such as a pregnancy during the follow-up period or whether those using the pill were 'healthy' individuals - ie not smokers or overweight.
"Overall, these findings are interesting, although more research needs to be conducted within this area before firmer conclusions can be drawn."