Amgen defined "event" as death, heart attack, stroke or "mini-stroke," unstable chest pain or heart failure requiring hospitalization, or the need for a procedure to restore bloodflow to the heart.
Side effects more frequent, but still rare, in patients treated with Repatha included neurocognitive problems, such as confusion, something the U.S. Food and Drug Administration has said should be monitored closely.
Read MoreWhy biotech investors may be hurting innovation
Scott Wasserman, Amgen's head of cardiovascular and metabolic therapies, told Reuters that the company does not believe there is a safety issue.
Neurocognitive side effects were also more common in the treatment arm of an 18-month, 2,300-patient trial of a rival PCSK9 drug being developed by Sanofi and Regeneron Pharmaceuticals.
The drug, Praluent, was shown to reduce the risk of cardiovascular problems from 3.3 percent for placebo patients to 1.7 percent for the treatment group. "Events" in this trial were defined only as death, heart attack, stroke and chest pain requiring hospitalization.
Read MoreHelp for people looking for medical breakthroughs
"Some of the endpoints in these trials are kind of soft, and they aren't prospective studies," said Dr. Anthony DeMaria, director of the cardiovascular center at the University of California San Diego who was not involved in the trials. "It's encouraging that the larger trials are likely to succeed, but we still need those trials."
Dr Marc Sabatine, lead investigator of the Repatha study, said data for both drugs "are very consistent ... it appears that cutting LDL by 61 percent translates to a roughly 50 percent reduction in cardiovascular events."
Numerous trials have shown that PCSK9 inhibitors significantly lower blood levels of "bad" LDL cholesterol, but investors expect widespread use will hinge on whether the drugs are proven to prevent death, heart attacks and other serious heart problems.
The experimental drugs are antibodies, given by injection, designed to target the PCSK9 protein that maintains LDL cholesterol in the bloodstream. They work differently from statins—pills, now available as low-cost generics, that block the liver's production of LDL cholesterol in the first place.
Both Amgen and Sanofi/Regeneron have filed for FDA approval of their drugs, based on trials showing that they lower LDL in patients whose cholesterol is not controlled by other drugs, those who cannot tolerate other drugs and people genetically predisposed to high cholesterol.
The FDA is slated to decide on Amgen's application for Repatha by Aug. 27, while the regulatory deadline for Praluent, also known as alirocumab, is July 24.
Health insurers are already bracing for the impact on drug spending. Express Scripts, the largest manager of prescription drug plans for U.S. employers, projects an annual cost as high as $10,000 per patient for PCSK9 drugs, which it says could be used for 10 million Americans.
Both Amgen and Sanofi/Regeneron do not expect definitive data on cardiovascular outcomes for their drugs until larger trials conclude in 2017. Pfizer, which has not yet filed for regulatory approval of its PCSK9 drug, said it expects outcomes data in a similar time frame.