US FDA flagged concerns in key Tavneos trials, causing Health Canada to advise doctors to stop starting the medication.
The warning issued after the FDA found data problems.
A medical journal has retracted a paper that used the trial data, noting the numbers were conflicting.
The New England Journal of Medicine has also withdrawn pivotal data from studies linked to Tavneos.
Regulators in both countries are examining the drug's safety and efficacy.
Amgen has still commented, maintaining the issue unresolved.
The developments trigger concerns for patients currently receiving the treatment.