US FDA flagged issues in key Tavneos trials, leading to Health Canada to advise doctors to pause starting the medication.
The warning released after the FDA identified data irregularities.
A medical journal has retracted a paper that used the trial data, noting the numbers were inconsistent.
The New England Journal of Medicine has also removed pivotal results from studies linked to Tavneos.
Regulators in both countries are assessing the drug's safety and efficacy.
Amgen has so far commented, leaving the issue unresolved.
The developments trigger concerns for patients currently receiving the treatment.