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Q. I have heard that St. John's Wort can prevent birth control pills from working correctly.  Is this correct?

That is true. St. John's Wort can decrease the efficacy of hormonal contraceptives. Also, it is known to cause menstrual irregularities in women not taking oral contraceptives.

St. John's Wort is an interesting herb. It's active ingredient is essentially a compound not unlike medicines such as Prozac or Zoloft, and accordingly we recommend not taking St. John's Wort if you are on any antidepressant or anti-anxiety medicine without letting your physician know. The medical literature stresses the fact that the active ingredient is in such low concentrations that most users of St. John's Wort are unlikely to experience any therapeutic benefit beyond a placebo effect.