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Figure  . More than a quarter of cancer patients may be at risk of serious drug interactions, primarily from treatments for comorbid conditions or supportive cancer care.

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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 (2007;99:592–580). However, chemotherapy agents were not the primary culprit, the team from Princess Margaret Hospital in Toronto, Canada, reports. Rather, medications received for comorbidities and cancer supportive care were more likely to be involved.

“It was noncancer 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 ambulatory adult patients treated in the follow-up medical oncology clinic of the hospital. Medication use was determined by a patient questionnaire and chart review. Potential interactions were identified with the Drug Interaction Facts software and pharmacology textbooks.

The team found 276 potential drug interactions in 109 patients (27%). Most of the potential interactions (77%) were of moderate severity, meaning the adverse effect would require medical treatment; 9% were of major severity, meaning the interaction could lead to permanent damage or death. Treating physicians were notified of potentially serious interactions.

Potential interactions with antineoplastic agents were most frequently seen with warfarin (15 cases), followed by hydrochlorothiazide (6 cases). But greater numbers of potential interactions were seen between noncancer medications. The most frequently noted possibility (19 cases) was between aspirin and angiotensin-converting enzyme (ACE) inhibitors or beta-blockers. The researchers also found 14 potential interactions of aspirin and corticosteroids and 13 of warfarin and corticosteroids. There were 9 potential interactions between the antiemetic prochlorperazine and ACE inhibitors and 7 between prochlorperazine/ranitidine and phenytoin.

The decentralized nature of modern health care may be part of the problem, Krzyzanowska says.

“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.

Interactions between different doctors are often limited, particularly if they practice in different hospitals, making it more difficult to keep up with the various medications patients may be taking. Electronic medical records can help, but even they may not be universally accessible.

Doctors need to keep drug interactions in mind, 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.

Patients also must take responsibility. Krzyzanowska suggests all patients keep an updated list of their 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.