Cancer Care Ontario says four hospitals are informing patients that they received lower than intended doses of the chemotherapy drugs cyclophosphamide and gemcitabine.
At total of 990 patients treated or being treated at London Health Sciences Centre, Windsor Regional Hospital, Lakeridge Health and Peterborough Regional Health Centre are affected.
As each case is patient-specific, Cancer Care Ontario says affected patients are being encouraged to discuss the possible effects of the underdosing with their oncologists.
The two medications were purchased by the hospitals from a supplier and Cancer Care Ontario and the hospitals are working with the supplier to find the cause of the error.
All four hospitals immediately removed the medications received from the drug manufacturer when the problem was discovered in late March, and informed the manufacturer of the error.
Each hospital has secured new supplies of the medications for all subsequent treatments, and Cancer Care Ontario says patients' treatment cycles will not be interrupted.