UW News

February 25, 2000

UW pharmacy professors publish doctors’ guide to drug interactions

Patients filling a prescription usually can rely on their pharmacist to warn of possible negative side effects caused by interactions with other prescriptions and over-the-counter medications that they may be taking.

But two top national experts on drug interactions from the University of Washington School of Pharmacy believe the health-care provider actually writing the prescription should be the first line of defense against such interactions.

Accordingly, Drs. Philip D. Hansten and John R. Horn, both professors of pharmacy, have published a pocket guide for health-care practitioners, called The Top 100 Drug Interactions: A Guide to Patient Management.

The guide is designed to be kept close at hand and consulted frequently. It is a condensed version of the authors’ major publication, Drug Interactions Analysis and Management.

It lists the three classes of drug interactions most likely to result in negative side effects. For Class 1 interactions, prescribers are advised to avoid the drug combination entirely; for Class 2, to use only if the benefit outweighs the risk; and Class 3, to assess the risk and take action if necessary.

The guide lists approximately 600 drug interactions and gives options for alternative, non-interacting drugs that practitioners can offer their patients as substitutes. A table lists 150 common drugs with the enzyme responsible for metabolizing each drug. By using the table, the practitioner may be able to predict potential drug interactions that have not yet been reported.

The authors say the guide doesn’t list every interaction, but it does include the most frequent and most dangerous ones. It lists some foods and beverages (such as grapefruit and alcohol) that interact with some medications, and includes both prescribed and over-the-counter medications.

“Our position is that drug interactions are almost 100 percent preventable,” said Hansten. “These interactions are predictable.”

For example, if a patient is taking either Zocor or Mevacor, both “statin” drugs used to lower cholesterol, the guide tells the health care provider to be cautious about prescribing the antibiotics erythromycin or Biaxin. Interactions could range from muscle pain and muscle damage, to pancreatitis and acute kidney failure in rare cases. The guide suggests alternative antibiotics that don’t interact in the same way with the cholesterol drugs, as well as other statin drugs that don’t interact with those antibiotics. As another alternative, it suggests the patient be temporarily taken off the cholesterol medication if the antibiotic therapy is short-term. The statin-antibiotic example is listed as a Class 2 interaction: the combination should be used only if the benefit outweighs the risk.

While the doctor should be the first line of defense, the authors caution that patients must be aware of all the medications, prescribed and over-the-counter, that they are taking. “We’re trying to get this data through to the users,” said Horn. “The patient is the only one who knows what drugs he or she is taking.”

“It’s a particular problem with the elderly, who may be taking a number of medications,” said Hansten. “The patient who’s taking five or more drugs can assume some interactions may be occurring. An estimated 15 percent of elderly patients are experiencing adverse effects.”

The American Hospital Association, notes Hansten, has introduced a major new initiative to reduce drug errors, in the wake of the Institute of Medicine’s report on medical mistakes in hospitals. “A guide like ours can help in that effort,” he said.

“Drug interactions are a very important focus for the School of Pharmacy,” said Horn, “from the basic science to clinical applications and public health. Three-quarters of our faculty are working in some way in the area of drug interactions.”

The pocket guide grew out of a handout that Hansten and Horn made for their pharmacy students.


For more information contact Dr. Philip Hansten at (206) 543-2579 or Dr. John Horn at (206) 543-1413.