US FDA flagged problems in major Tavneos trials, raising Health Canada to advise doctors to stop starting the medication.
The warning issued after the FDA identified 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The New England Journal of Medicine has also withdrawn pivotal findings from studies connected to Tavneos.
Regulators in both countries are assessing the drug's safety and efficacy.
Amgen has still commented, leaving the issue unresolved.
The developments trigger concerns for patients currently receiving the treatment.