Research: PFRUNDER and colleagues,

Listed in Issue 106


PFRUNDER and colleagues, Department of Clinical Pharmacology and Toxicology, Women's Hospital, University Clinic Basel / Kantonsspital, Basel Switzerland, have studied the interaction of St John's wort with low-dose oral contraceptive pills.


Bleeding out of turn and even unwanted pregnancies have been reported by women who are taking St John's wort along with oral contraceptive. This study aimed to investigate this phenomenon.


18 healthy women were treated with a low-dose oral contraceptive (ethinyloestradiol plus desogestrel) for one cycle (control cycle), the oral contraceptive plus 300 mg St John's wort extract twice daily for cycle A, and three times daily for cycle B in a randomized crossover fashion. Ovarian activity was assessed by measuring follicel maturation and serum oestradiol and progesterone levels. The number of breakthrough bleeding episodes and the pharmacokinetics of the contraceptive were recorded.


The simultaneous administration of oral contraceptive and St John's wort did not influence ovarian activity. However significantly more bleeding episodes out of turn were recorded (p < 0.001). The metabolism of ethinyloestradiol was unchanged, but the peak concentrations and lifetime of 3-ketodesogestrel decreased significantly in the interaction with St John's wort.


There was no evidence of ovulation during concomitant treatment with oral contraceptive and St John's wort, but intracyclical bleeding increased significantly. Bleeding irregularities may adversely affect compliance with oral contraceptive therapy and, together with the reduction in serum desogestrel levels, increase the risk of unwanted pregnancies.


Pfrunder A, Schiesser M, Gerber S, Haschke M, Bitwer J, Drewe J. Interaction of St John's wort with low-dose oral contraceptive therapy" a randomized controlled trial. British Journal of Clinical Pharmacology 56 (6): 683-690, Dec 2003.

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