Clinicians and patients should beware of possible decreases in the systemic bioavailability of conventional drugs when taken concomitantly with St John 's wort
OBJECTIVE To determine the method ological quality of clinical trials that examined possible interactions of St John 's wort with conventional drugs , and to examine the results of these trials .
Interaction of St John's wort with low-dose oral contraceptive therapy: a randomized controlled trial.
AIMS Breakthrough bleeding or even unwanted pregnancies have been reported in women during concomitant therapy with oral contraceptives and St John 's wort extract .  The aim of the present study was to investigate the effects of St John 's wort extract on oral contraceptive therapy with respect to ovarian activity , breakthrough bleeding episodes and the pharmacokinetics of ethinyloestradiol and 3-ketodesogestrel .  METHODS Eighteen healthy females were treated with a low-dose oral contraceptive ( 0.02 mg ethinyloestradiol , 0.150 mg desogestrel ) alone ( control cycle ) or combined with 300 mg St John 's wort extract given twice daily ( cycle A ) or three times daily ( cycle B ) . Ovarian activity was assessed by measuring follicle maturation and serum oestradiol and progesterone concentrations . The number of breakthrough bleeding episodes and the pharmacokinetics of ethinyloestradiol and 3-ketodesogestrel  were assessed under steady-state conditions . RESULTS During concomitant administration of low-dose oral contraceptive and St John 's wort , there was no significant change in follicle maturation , serum oestradiol or progesterone concentrations when compared with oral contraceptive treatment alone . However , significantly more subjects reported intracyclic bleeding during cycles A ( 13/17 ( 77 % ) , P < 0.015 ) and cycle B ( 15/17 ( 88 % ) , P < 0.001 ) than with oral contraceptives alone ( 6/17 ( 35 % ) ) . The AUC(0,24 h ) and Cmax of ethinyloestradiol remained unchanged during all study cycles , whereas the AUC(0,24 h ) and Cmax of 3-ketodesogestrel  decreased significantly from 31.2 ng ml-1 h to 17.7 ng x ml-1 h ( 43.9 % ; 95 % confidence interval ( CI ) -49.3 , -38.5 , P = 0.001 ) and from 3.6 ng x ml -1 to 3.0 ng x ml -1(17.8 % ; CI -29.9 , -5.7 , P = 0.005 ) , respectively , during cycle A and by 41.7 % ( CI -47.9 , -35.6 ; P = 0.001 ) and by 22.8 % ( CI -31.2 , -13.3 ; P < 0.001 ) during cycle B respectively , compared with the control cycle . CONCLUSIONS There was no evidence of ovulation during low-dose oral contraceptive and St John 's wort extract combination therapy , but intracyclic bleeding episodes increased . Bleeding irregularities may adversely effect compliance to oral contraceptives and together with St John 's wort-induced decreases in serum 3-ketodesogestrel concentrations , enhance the risk of unintended pregnancies
Clinicians and patients should beware of possible decreases in the systemic bioavailability of conventional drugs when taken concomitantly with St John 's wort
OBJECTIVE To determine the method ological quality of clinical trials that examined possible interactions of St John 's wort with conventional drugs , and to examine the results of these trials .