US FDA flagged problems in key Tavneos trials, causing Health Canada to advise doctors to stop starting the medication.
The warning released after the FDA found data irregularities.
A medical journal has retracted a paper that used the trial data, saying the numbers were inconsistent.
The New England Journal of Medicine has also removed pivotal findings from studies linked to Tavneos.
Regulators in both countries are examining the drug's safety and efficacy.
Amgen has not yet commented, leaving the issue unresolved.
The developments trigger concerns for patients as of now receiving the treatment.