As many as 74 percent of patients taking a widely prescribed class of oral cancer drugs are also on other medications that could decrease the effectiveness of the cancer treatment and increase the toxic side effects, according to a new study.

Researchers at Medco Health Solutions Inc. found that 23 to 57 percent of patients taking on targeted cancer medication were also prescribed drugs that may have dampened the effect of the cancer treatment, and that 24 to 74 percent of the cancer patients were on other drugs that may have added toxic side effects when used simultaneously with the cancer therapies.

The findings reveal that the majority of the cancer medications were typically prescribed by an oncologist, while the other drugs were prescribed by a primary physicians or non-cancer specialists.

“Oncologists are not always aware of other medications prescribed by other doctors and vice-versa, which can pose a real hazard for their patients on oral cancer therapies,” said co-author Steven Bowlin, of the study and senior director at Medco’s research division, in a statement.

"More communication needs to take place across all doctors that are prescribing for the patient," said Bowlin, who is presenting the study findings on Friday at the American Society for Clinical Pharmacology and Therapeutics meeting in Washington, according to Reuters.

Some of the drugs that can cause adverse “drug-drug interactions” are some commonly used heartburn drugs called proton pump inhibitors (PPIs), like Nexium and Prevacid, steroids, blood pressure and heart drugs called calcium channel blockers, and antibiotics and antifungal treatments.

The Medco drug interaction research evaluated pharmacy claims of about 11,600 patients who had been prescribed any of nine oral drugs called kinase inhibitors used to treat a wide range of cancers, which include Gleevec and Tasigna from Novartis; Pfizer Inc's Sutent; Nexavar from Onyx Pharmaceuticals Inc and Bayer AG; Tarceva, sold by Roche Holding and Japan's Astellas Pharma Inc; Sprycel from Bristol-Myers Squibb Co; and GlaxoSmithKline Plc's Tykerb.

Researchers found that 74 percent of patients who were taking Glaxo's kidney cancer drug Votrient were also prescribed a drug that could boost its toxic side effects.

About 43 percent of patients taking a leukemia drug Gleevec were also on another prescription that could decrease its efficacy, and 68 percent were taking something that could potentially raise the toxicity level.

More patients on Tarceva were also taking another drug that could diminish its efficacy than any other cancer treatment patients at 57 percent.

Researchers said that Sutent patients had the lowest incidence of potentially hazardous drug combinations with 23 percent taking a drug that could reduce its effectiveness and 24 percent taking another drug that could increase its toxicity.

"The fact that one-quarter to 75 percent of patients on these oral drugs may not be getting the full benefit of their treatment or may in fact be putting their health at further risk because of another medication they are taking is concerning," Dr. Milayna Subar of Medco's Oncology Therapeutic Resource Center said in a statement.

Medco, a pharmacy benefit manager in the process of being acquired by rival Express Scripts Inc, says that it offers a Drug Utilization Review program that allows oncology pharmacists to alert doctors about potentially troubling medication interactions.

"As we learn more about how to optimize treatment with cancer drugs, we are in a better position to share this knowledge with our patients and other clinicians through our Therapeutic Resource Centers, which can alert clinicians to these potential interactions, and so we can collaborate with them to seek therapeutic alternatives with less interaction potential whenever possible," co-author Dr. Eric Stanek, and vice president of research, said in a statement.