Progestogen fell over 40 percent and breakthrough bleeding doubled
Added to a low-dose combined pill, it cut progestogen exposure at both doses tested while leaving the oestrogen untouched. Irregular bleeding rose from 6 of 17 women to 13, then 15.
What the study found
Progestogen down more than 40 percent, breakthrough bleeding doubled
Adding St John's Wort to a low-dose combined pill cut exposure to its progestogen by more than 40 percent at both doses tested, while leaving the oestrogen completely untouched. Irregular bleeding rose from 6 of 17 women to 13, and then to 15.
The caveat · Too few women to rule out ovulation
Seventeen women, and although none ovulated, a study this small could not have detected a modest rise in ovulation if there were one. Bleeding was recorded by diary. The hyperforin content of the preparation was never reported, and that is the variable later work shows decides the size of the effect.
What we don't know
Whether St John's Wort lets ovulation break through a combined pill is unsettled. In both trials the pill's hormone levels fell and bleeding rose, but they split on ovulation, and none measured pregnancy.