The Saint John Regional Hospital is now mixing its own chemotherapy drugs after some cancer patients were given watered down doses over the past year.

A total of 1,176 cancer patients in New Brunswick and Ontario received a diluted mixture of cyclophosphamide, used to treat some cancers, such as breast and non-Hodgkin's lymphoma, Cancer Care Ontario announced on Tuesday.

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Dr. John Dornan, chief of staff at Horizon Health Network's Saint John zone, says the Saint John Regional Hospital is now mixing its own chemotherapy drugs. (CBC)

Of those, 186 were treated at the Saint John Regional Hospital, said Dr. John Dornan, chief of staff for the Horizon Health Network's Saint John zone.

The hospital was notified of the error on March 28.

"Such incidents are rare and, when they happen, we have processes in place to address issues while being responsive, open and transparent," Dornan said in a statement on Wednesday. "We are taking this matter seriously."

Risk to patients unclear

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The Saint John Regional Hospital treated 186 people with the diluted mixture of cancer drugs. (CBC)

The affected patients, who are mainly from the Saint John and Fredericton areas, are being contacted by registered mail and by phone to inform them they received lower than intended doses, he said.

It's unclear at this point how the diluted drugs will affect individual patients, said Dornan. Each case is patient-specific, he said.

The risks will also depend on whether they received a small or large amount of dilution, said Dornan. The dilution estimates ranged from three to 20 per cent, Ontario officials have said.

Affected patients are being asked to contact their doctor to make an appointment to determine how they were affected, said Dornan.

Working with supplier

Meanwhile, the hospital is working with the supplier, who produces and labels the chemotherapy drugs, to determine why the premixed bags contained too much saline solution.

The supplier is Marchese Health Care in Hamilton, Ont., Cancer Care Ontario officials have said.

The error was identified when a pharmacy technician at one of the Ontario hospitals noticed a discrepancy between what a patient was receiving and what was supplied, Dornan said.

All of the cyclophosphamide in question has been removed and the Saint John Regional Hospital has secured appropriate supplies for subsequent treatment, officials said.

Patients' treatment cycles will not be interrupted during this transition, they said.

The Saint John Regional Hospital is the largest tertiary care hospital in New Brunswick.