Story highlights Close to 1,200 patients were affected at five hospitals

The underdosing ranged from 3% to 20%

The drugs were bought by the hospitals from a supplier

Close to 1,200 patients in Canada got lower-than-intended doses of chemotherapy drugs, officials said Tuesday, in a case that is sure to upset those patients and raise concerns about how potentially life-saving drugs are handled.

Cancer Care Ontario, a government agency, reported that 990 patients were affected at four hospitals: London Health Sciences Centre, Windsor Regional Hospital, Lakeridge Health and Peterborough Regional Health Centre.

Another 186 patients were underdosed at Saint John Regional Hospital in New Brunswick, said hospital spokeswoman Janet Hogan.

The drugs in question were bought by the hospitals from a supplier that produces and labels the medications, according to Cancer Care Ontario.

The supplier was identified as Marchese Hospital Solutions, a company spokesman said.

"We are, of course deeply concerned whenever any question is raised about the quality of our work," the company said in a statement. "Our preliminary investigation of this issue leads us to be confident that we have met the quality specifications of the contract we are honored to have been awarded."

The problem, which involved cyclophosphamide and gemcitabine, was discovered late last month.

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Cyclophosphamide is typically used to treat cancer of the ovaries, breast, blood and lymph system, while gemcitabine can be used in the treatment of breast, ovarian, pancreatic and lung cancers.

"It's important to note that chemotherapy preparation and delivery is a complex process and as a result of this complexity, there are sources for potential error," Dr. Carol Sawka, vice president of clinical programs and quality initiatives at Cancer Care Ontario, said in a statement.

"In Ontario, we have put in many steps to minimize these potential sources of error and we will continue to ensure that patient safety and high quality care are the focus and the strength of the system."

Sawka provided more details on the case when she spoke to CNN and CNN affiliate CTV

She said the problem with the drugs, which are administered intravenously, was first identified by a pharmacy technician.

"We're working with the hospitals and will be working with the supplier to really go over this very, very carefully and try to determine the cause of the error," Sawka told CNN.

She declined to speculate on what the consequences might be for patients, stressing that each person and each treatment is different.

Speaking to CTV, Sawka estimated that the range of underdosing was from 3% to 20% less than what it should have been.

Affected patients and their families are being notified of the error this week.

"We want to learn from it. We want to make sure it never happens again," Sawka told the affiliate.