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More than a quarter of cancer patients may be at risk of potentially serious drug interactions, according to a recent study in the Journal of the National Cancer Institute. However, chemotherapy agents were not the primary culprit, the team from Princess Margaret Hospital in Toronto, Canada, reports.

Rather, medications received for conditions other than cancer or for the side effects of cancer treatment were more likely to be involved.

"It was non-cancer drugs interacting with each other," explains researcher Monika K. Krzyzanowska, MD, MPH, assistant professor in the department of medicine at the University of Toronto and staff physician in the department of hematology and oncology at Princess Margaret Hospital. "In cancer, most of the focus has been on chemotherapy interacting with other agents, and there was some of that, but the majority was between the other drugs -- what they were getting for blood pressure or diabetes -- or those medications interacting with the medications for supportive cancer care, like steroids."

Krzyzanowska and colleagues studied 405 adult patients treated in the follow-up medical oncology clinic of the hospital. They asked participants about the medicines they were taking, and reviewed their medical charts. They used a software program and pharmacology textbooks to flag potential drug interactions.

The team found 276 potential drug interactions in 109 patients (27%). Most of the potential interactions (77%) were of moderate severity, meaning the problem that could result would require medical treatment. Nine percent were of major severity, meaning the interaction could lead to permanent damage or be life-threatening. The research team notified the participant's doctor of any serious potential interactions.

The drugs that were most likely to interact with chemotherapy drugs included the blood thinner warfarin (Coumadin) and the water pill hydrochlorothiazide, which is used to reduce fluid retention and treat high blood pressure.

But greater numbers of potential interactions were seen between non-cancer medications. The most frequently noted possibility was between aspirin and ACE inhibitors or beta-blockers, drugs used to treat high blood pressure and other heart problems. The researchers also found potential interactions between aspirin and corticosteroids (often used to treat side effects of chemotherapy) and between warfarin and corticosteroids. There were also potential interactions between the anti-nausea drug prochlorperazine (Compazine) and ACE inhibitors, and between prochlorperazine/ranitidine and the seizure medication phenytoin (Dilantin).

The decentralized nature of modern health care may be part of the problem, Krzyzanowska says. Interactions between the different doctors a patient is seeing is often limited, particularly if they practice in different hospitals, making it more difficult to keep up with the various medications the patient may be taking.

"Family doctors or cardiologists are most likely to be prescribing blood pressure medications, but they are the least likely to be referring a patient for cancer treatment," she explains.

Electronic medical records can help, but even they may not be universally accessible.

That means patients really need to keep track of all the medicines they take.

Krzyzanowska suggests keeping an updated list of your medications to show each doctor. She also recommends sticking to a single pharmacy for all prescriptions, if possible, since a pharmacist with access to all prescription information may be able to flag potentially dangerous combinations.

Of course, doctors need to keep drug interactions in mind, too, Krzyzanowska says.

"Be aware that this is an issue, so periodically review the medications with patients, and when you're adding things, think about what they're already on," she advises.

Citation: "Potential Drug Interactions and Duplicate Prescriptions Among Cancer Patients" Published in the April 18, 2007, Journal of the National Cancer Institute (Vol. 99, No. 8: 592-600). First author: Rachel P. Riechelmann, MD, Princess Margaret Hospital, Toronto.