The disclosure added to safety questions already clouding the mediation meant to treat heart failure made by the Johnson & Johnson unit Scios, The New York Times reported.
With the two additional deaths, Scios said seven patients died during the drug trial. But the company said that did not change safety assumptions about Natrecor.
Neither Scios nor the principal investigator, Dr. W. Franklin Peacock of Cleveland Clinic, disclosed how the deaths were omitted from results reported in October in The Journal of Emergency Medicine.
A Cleveland Clinic spokeswoman said Peacock believed the deaths, which were discovered in September, were accidental and unrelated to the drug.
"When you're talking deaths in clinical trials, mistakes are not an option," Dr. Arthur Caplan, a University of Pennsylvania medical ethicist, told the Times.