Background:

Millions of women worldwide use exogenous hormones as oral contraceptives or hormone replacement therapy. Still, time-dependent and long-term consequences of exogenous hormones on stroke risk remains unclear.

Methods:

We examined the association between self-reported oral contraceptive and hormone replacement therapy use and stroke risk in 257 194 women from the UK Biobank, born between 1939 and 1970. Outcomes included any type of stroke, ischemic stroke, intracerebral hemorrhage, and subarachnoid hemorrhage. Exposures were analyzed as time-varying variables in Cox regression models.

Results:

During first year of oral contraceptive use, an increased event rate of any stroke was observed, while the hazards were found to be comparable during remaining years of use, compared with nonusers. Similarly, first year of hormone replacement therapy use was associated with higher hazard rates of any stroke, as well as cause-specific stroke, including ischemic stroke and subarachnoid hemorrhage, which remained increased for any stroke during remaining years of use, and after discontinuation.

Conclusions:

Oral contraceptive use and hormone replacement therapy were associated with an increased risk of stroke, especially during the first year of use, possibly due to immediate changes in hemostatic balance. This study provides new insights on the effects of hormone exposure on stroke risk and provide evidence of not only an overall risk but also a pronounced effects seen in the beginning of treatment.