A Quick Look at Herbal Supplements and Prescription Drug Interactions

Patients with HIV regularly used St. John’s wort to ease depression until just recently when it was found that St. John’s wort dramatically reduced blood levels of Crixivan.  Crixivan is an important HIV drug and protease inhibitor.  At the low levels of concentration, AIDs patients are likely to become resistant to Crixivan.

The Food & Drug Administration began to alert doctors and request that Merck, the maker of indinavir, add a warning about St. John’s wort to the drug’s label.

There are as many as 40% of adults in the U.S. who regularly use herbs, vitamins, and other supplements and about 15 million take prescription drugs at the same time.  Many of these people never tell their doctors of the mixtures.

There are no wide-ranging clinical trials definitely linking herbal supplements to interactions with drugs, however, several case studies from the past 30 years suggest many interactions may exist.  Leading herbal supplements that may interact negatively with prescription drugs include:

• St. John’s wort


• Melatonin


• Kava


• Garlic


• Garlic extracts

St. John’s wort may also interact with an array of prescription drugs because it is broken down by an enzyme that also metabolizes about half of existing over-the-counter and prescription drugs.  There is some evidence that St. John’s wort reduces levels of:

• Lanoxin (digoxin) a drug that treats congestive heart failure


• Coumadin (warfarin), a blood thinner


• Bronchodilator drugs for asthma


• Tegretol (carbamazepine), an anticonvulsant

In addition to drug interactions, some herbal products appear to be addictive, which intensifies the effect of certain drugs.  The most worrisome are a few popular herbal products such as:

• Garlic


• Ginkgo

The use of garlic and ginkgo appear to increase the effect of the blood thinner Coumadin, which has led to severe bleeding in several case studies.

Patients who regularly take Zoloft or Paxil should stay away from St. John’s wort that is marketed as a substitute for prescription antidepressant drugs.  People on Xanax or Valium for anxiety should not take kava, an herb marketed as a relaxant.

Of course there are arguments from the Herbal product makers who say their herbs are extremely safe and that claims to the contrary are unfounded and cause unnecessary fears. 

Consumers should realize that herbal supplements work much like drugs.  Many people are leary of taking drugs prescribed by their doctor and will ask probing questions, then venture onto the local health-food store where a high-school diploma person will say, “Take this to ease your headaches,” and they immediately say “Okay,” and purchase the product.  People have the general idea that if something is labeled as being “natural,” this means it is a safe product and “prescription” could mean something toxic for them.

The problems with identifying herb-drug interactions include:

·        There is a general lack of knowledge about dangerous interactions

·        Product labels on herbal supplements usually are not helpful

·        The FDA does not push drug companies to investigate herb-drug interactions

·        Herbal product makers are not required to advertise possible adverse reactions

·        Under federal law, herbal products do not need to prove safety and efficacy

·        Herbal product makers do not need to conform to good manufacturing practices

The PDA does plan to establish standards for herbal product manufacturers.

Experts recommend:

• Consumers, particularly those taking prescription drugs for chronic conditions, should consult a doctor, herbalist, or pharmacist about possible interactions before taking any kind of supplement

Source:  Alger A. Dangerous Cocktail: Herbals and Drugs. Business Week [serial online]. May 08, 2000;2:E12. Available from: MAS Ultra - School Edition, Ipswich, MA. Accessed August 17, 2008.