Humans have been learning about the diverse effects of ingesting plants throughout our evolution. We have, by trial and error, found both good and bad effects that are related to specific plants, some of which we use as food, and others that are used for therapeutic purposes. The introduction of synthetic and highly purified drugs is an extremely modern development. As researchers observe the interactions that drugs have with common foods and herbs, surprises continue to surface. For example, less than 10 years ago it was found that drinking grapefruit juice increases the serum drug concentration when patients take certain drugs.(23) This effect, which can last for up to 24 hours after consumption, is now thought to inhibit specific enzymatic activities responsible for breaking down the drugs.(24) Similarly, both avocado and leafy vegetables that are high in vitamin K can diminish the effectiveness of blood-thinning drugs.(25) These concerns are not widely known by the public, but now that medical professionals are aware of these effects, they can routinely monitor their patients to assure effective treatment.
Similar information has surfaced about some of the herbs that we use. For example, we now know that the use of an extract of St. John's wort (Hypericum perforatum) may cause certain prescription medications to be eliminated more quickly,(26, 27) leading one researcher to caution that, "As with grapefruit juice, a food product, physicians should also be aware of potential drug-herb interactions."(26) In response to this new information, the American Herbal Products Association has recommended that products containing St. John's wort be labeled to suggest that the advice of your prescribing physician be requested if you are taking any prescription drugs. Speculation on the exact mechanism of St. John's wort has led to reports that the use of this herb might affect oral contraceptives, leading to ineffectiveness and unwanted pregnancies. To date, there have been no reports of any such actual occurrence. Nevertheless, women taking oral contraceptives such as ethinyl estradiol and desogestrel should be aware that, if you experience break-through bleeding, you might experience a reduction in protection against pregnancy.
As can be seen by the above examples, the effect of a drug can be either increased or decreased in the presence of other factors in the diet, including herbal use. Although it is likely that most such factors have little or no influence on drug metabolism, continued research will add to our knowledge of such interactions and responsible food and supplement manufacturers will be expected to inform their customers of any new findings. There is now an ongoing interest in other drugs that are suspected of interacting with certain specific herbs, with most contemporary emphasis on the use of herbs with blood-thinning drugs such as warfarin. Although the current concerns are either conceptual or based on isolated and inconclusive reports, it is advisable to inform your prescribing physician or pharmacist that you are using herbs when undergoing any drug therapy. As close monitoring of the effect of warfarin is an established standard of medical practice, this additional information will assist your physician in maintaining good supervision of your drug levels. In order to understand the potential for an herbal product to interact with prescription drugs, it may also be useful to consult with a qualified herbal expert. You can find a regional list of herbalists the American Herbalists Guild's professional member referral list at http://www.americanherbalistsguild.com
, or your local acupuncturist at the American Association of Oriental Medicine http://www.aaom.org
. A similar database of naturopathic physicians is maintained by the American Assoc. of Naturopathic Physicians at http://naturopathic.org