TORONTO - Many cancer patients are at risk for potentially dangerous drug interactions because of the number of different medications they take for multiple conditions, say Canadian researchers, who caution that steps must be taken to avoid these dicey combinations.
In a study appearing Wednesday in the Journal of the National Cancer Institute, researchers at Princess Margaret Hospital in Toronto found that almost 30 per cent of 405 cancer patients studied were taking drugs that put them at risk for at least one adverse drug interaction.
At least nine per cent of the interactions could have had severe effects -- including being potentially fatal -- and 77 per cent were of moderate severity and could have resulted in serious health problems, the study showed. About eight per cent of patients received duplicate medications.
"Cancer is really a disease of the elderly, and a lot of the patients that we are seeing don't just have cancer but have a lot of other medical problems,'' said Dr. Monika Krzyzankowska, a medical oncologist at Princess Margaret and senior author of the study.
Those other conditions include high blood pressure, cardiovascular disease and diabetes, any of which may require treatment with a variety of medications, she said. "And, in fact, what we did find was that the patients who were at higher risk were the ones who were getting'' a number of prescriptions.
One of the most common drugs to cause problems is Coumadin (known generically as warfarin), a blood thinner given to patients to prevent strokes due to blood clots.
"Coumadin is a very tricky drug, it interacts with everything, even things you eat,'' said Krzyzankowska, noting that it doesn't mix well with steroids, such as prednisone, which are frequently given to cancer patients.
"And sometimes it's unpredictable which way the interaction will go,'' she said. If warfarin levels get too high, blood becomes overly thinned and creates a risk of bleeding; too low, and a blood clot may form that could lead to a stroke.
The anti-seizure drug phenytoin also can interact adversely with cancer-related drugs, "but there were other less common interactions that we came across,'' she said. "We had close to 300 combinations of drugs that had the risk of interactions.''
With most cancer patients seeing several health-care providers, it's not surprising that their list of drugs might contain some conflicts.
"Most people who have oncologists also have family doctors and other specialists,'' Krzyzankowska explained. "They're seeing a cardiologist, they might be seeing an endocrinologist for their diabetes and they're getting medications from multiple doctors.''
Some patients keep a list of their medications or bring the doctor a bag of all their pills, she said. "But sometimes they come and they say: `Oh, I don't remember. I'm on some blood pressure medication.' But the type of blood pressure medication matters in terms of this issue.''
With each visit to a different physician, drugs may be added or subtracted from the list -- "and eventually God knows what cocktail you end up on,'' she said.
"The message to me from this (study) is if you are seeing multiple providers of care, multiple types of doctors and they're all changing your medications, you should always be aware of everything you're on, so that you can tell them: `I'm on this medication and I'm on that medication.'''
Rita Kwong, Princess Margaret's pharmacy site operations manager, said the percentage of risky medication combos identified in the study was not surprising "because we know in the pharmacy profession that drug interactions are quite common.''
"What we need to make sure is that the health-care providers are aware of the potential drug interactions that a patient may experience and see if anything can be done to either mitigate the risk by changing a drug therapy or by having very close monitoring of a patient.''
The challenge in avoiding bad drug interactions is that no electronic database of patient records and prescriptions exists that doctors at different hospitals, clinics or offices can access. And if a patient has prescriptions filled at more than one pharmacy, that makes ensuring a safe drug profile even more difficult.
"When a patient visits a drug store, if they attend the same drug store all the time, then that drug store should have a fairly complete history of medications that are dispensed to the patient.'' Kwong said. "And when possible, if information is solicited from the patient, the pharmacy would also record some of the over-the-counter medications that the patient may be taking concurrently.''
"The difficulty is when patients visit multiple drug stores and also see multiple physicians.''
Kwong advises patients to:
- Always carry an up-to-date list of medications and their dosages, including over-the-counter and alternative therapies.
- Try to have prescriptions filled by the same pharmacy: electronic software will alert the druggist of potential drug interactions.
- Make sure to keep different doctors or pharmacists apprised of any changes in prescriptions.
"There needs to be better communication between all of the person's health-care providers, and the patient needs to take ownership of their own medication records,'' she said.
In an accompanying editorial, Dr. Peter Norton of the University of Calgary and Ross Baker of the University of Toronto agree that a better system for tracking patients' medications is critical.
"We believe that such co-ordination and communication have the potential to substantially reduce the incidence of and mitigate possible harm from the potential drug interactions that have been discovered by (the researchers).''
"In other high-risk industries, such as aviation and the offshore oil industry, many errors result from poor co-ordination and communication, and in these areas, strategies to improve team co-ordination and communication have led to reductions in incidents.''
