NEW YORK (Reuters Health) - A potential drug interaction is present in 27 percent of cancer patients receiving chemotherapy for solid tumors, new research shows. In most cases, the interaction involves medications used to treat co-existing illness.

By contrast, the findings indicate that duplicate medications are uncommon in this patient population. The results are published in the Journal of the National Cancer Institute.

Dr. Monika K. Krzyzanowska, from the Princess Margaret Hospital in Toronto, and colleagues reached these conclusions after surveying 405 adult cancer outpatients about medications taken in the previous 4 weeks. Potential drug interactions and their severity were determined by using a software program called Drug Interaction Facts.

A total of 276 potential drug interactions were recorded in 109 patients. The majority of interactions were classified as moderate, although 25 were considered severe.

Eighty-seven percent of the interactions did not involve the chemotherapy drugs, but instead involved drugs, such as the anticoagulant warfarin, high blood pressure agents and steroids, used to treat co-existing conditions.

The likelihood of a potential drug interaction increased by 40 percent for each addition medication used, and was 8.4-times more likely with drugs used to treat other conditions. The presence of brain tumors increased the risk of a potential interaction as well.

Eight percent of patients received duplicate prescriptions, most often with steroids, proton pump inhibitors and benzodiazepines, the report indicates.

As noted in a related editorial, the clinical consequences of these potential drug interactions are not known.

Because we don't know the extent of harm cause by these potential interactions, further research is needed "to understand the unintentional harm experienced by cancer patients as a result of their medication treatments," write Dr. Peter G. Norton, from the University of Calgary, and Dr. G. Ross Baker, from the University of Toronto.

The findings also suggest "that to understand the burden of adverse events for cancer patients, it will be necessary to track these patients across the continuum of care and to ensure that potential adverse events are identified, reported, and investigated."