Dietary Supplements

About one in four people who take prescription medication also use such products as vitamins, minerals, amino acids, herbs and/or related ingredients. Despite the therapeutic nature of some of these supplements, they may also either enhance or antagonize the effects of prescription drugs.

A recent study in the American Family Physician journal reviewed the dietary supplement-drug interaction situation using Medline, EMBASE, CINHAHL and Drug Interaction Facts databases focusing on patients who were using supplements to augment their treatment of chronic disease. The most frequent diseases treated by dietary supplements (DS) include asthma, insomnia, depression gastrointestinal problems, pain, memory problems and menopausal symptoms.

DS-drug interactions can be pharmacodynamic or pharmacokinetic. Pharmacodynamic interactions occur when the intrinsic action of a DS augments or antagonizes the action of the pharmaceutical drug. Pharmacokinetic interactions result from changes in metabolism. Absorption or protein binding of the drug or the DS thus resulting in a more pronounced or diminished pharmacologic activity.
The following information was extracted from this study:

St. John’s Wort, for example, increases the metabolism of warfarin leading to diminished levels. Ginkgo does not interact with warfarin directly, but rather has demonstrated antiplatelet activity. The authors report that vitamin E shows no increased risk of bleeding and that cranberry juice was not shown to affect coagulation in controlled studies. Results from a clinical study of 16 patients indicated that fish oils did not affect the coagulation status of patients taking warfarin. 

The authors noted that it was well known that folic acid affected warfarin activity and went on to state that grapefruit juice is known to significantly affect the metabolism of many drugs. American ginseng is known to decrease warfarin serum levels in humans resulting in less anticoagulation. The authors further stated that it is good advice for that, given the narrow therapeutic window for the efficacy of warfarin, that patients taking dietary supplements should be monitored closely whenever they initiate or stop taking any supplement until the effect of the interaction has been determined.

For patients taking cardiovascular medications, St. John’s Wort is associated with the most interactions. It decreases the serum levels of verapamil and statins. In one study St. John’s Wort decreased digoxin level by 25% and in another study ginseng was reported to cause an increase in serum digoxin levels.

For patients on psychiatric medications, St. Johns Wort is believed to affect serum serotonin levels, although no data to this effect exists. According to this author’s review findings, it has been associated with serotonin syndrome in patients taking a selective serotonin reuptake inhibitor (SSRI); it decreases serum levels of psychiatric medications metabolized by cytochromes P450 and it affects serum levels of benzodiazepines and tricyclic antidepressants.

Ginseng, chromium and psyllium all have hypoglycemic effects although the effect is unpredictable. Psyllium can diminish or slow the absorption of many drugs and as a rule should be taken several hours before or after administration of medications.

To conclude, the authors suggest that all patients be asked about their use of dietary supplements and be advised about the safety and effectiveness of the products they are using or considering using. Resources for the dietitian and other health professionals exist on the web and in pocket-size books on herb-drug interactions and drug-supplement interactions.

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