Page 69 - 《中国药房》网络版 2018年6月第2期科普版

《中国药房》网络版 2018 年 6 月第 2 期科普版 Jun No.2 2018 China Pharmacy online Popular Science Edition

Oral Contraceptives and St John's Wort

Assessing the clinical importance of drug–drug interactions involving

herbal medications can be problematic.

There is a general lack of high-quality studies, as shown by a recent

systematic review of drug interactions of warfarin with herbal medicines.

Just 9 randomized controlled trials were identified out of hundreds of

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published studies, and even these studies had some flaws. In addition,
it is often difficult to precisely assess the potency and content of herbal

products. It is possible, for example, that one type of ginseng might

interact with a particular drug while a different ginseng product might

not.

For St John’s wort, however, sorting out which drug interactions are real
is somewhat more straightforward. First, St John’s wort can be

standardized based on hypericin content. Moreover, there is a substantial
amount of evidence to show that St John’s wort acts as an inducer of CYP3A4

and, to a lesser extent, CYP2C19 and CYP2C9. St John’s wort also induces

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the transporter P-glycoprotein (ABCB1). The induction of these enzymes
and transporters could reduce the efficacy of a variety of other

medications, including oral contraceptives. Cases of breakthrough

bleeding and unintended pregnancy have been reported in women taking St

John’s wort with oral contraceptives, but it is difficult to assess such

case reports because bleeding and pregnancy can occur whether or not the

woman is taking interacting drugs.

A comprehensive evaluation of the evidence sup- porting the interaction

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