Did you know that St. John's wort can make your birth-control pills and many other drugs ineffective? The supplement causes the liver and intestines to produce higher levels of an enzyme that processes many drugs, causing them to move through your system more quickly. As a result, less of the drug may reach the bloodstream. Goldenseal, which is often packaged with echinacea to treat colds and gastrointestinal problems, can have the opposite effect. It enhances certain drugs, such as antidepressants and beta blockers, which can lead to toxic blood levels.

Read more about risky herb-drug interactions, and take a look at our Interaction Checker (subscribers only) to find out which natural medicines interact with your prescription drugs.