Novartis announced on Friday that its experimental drug, pelacarsen, widely assumed to be the vanguard of a new era in preventing heart attacks, strokes and cardiovascular deaths, had failed. The announcement gave no further details, but it came as a shock to the field of cardiology.

“Oh no — oh no,” said Dr. David Maron, president of the American College for Preventive Cardiology. “This is a huge blow. I hardly know what to say.” Dr. Harlan Krumholz, a Yale cardiologist, exclaimed: “This is extremely disappointing news. We’ve all been looking forward to celebrating.”

The drug blocks a protein called lipoprotein (a) that is a powerful risk factor for heart disease. Every study short of a randomized clinical trial indicated that lowering Lp(a) would protect people from developing the disease. Cardiologists widely expected that Lp(a) drugs would be the missing link in attempts to prevent heart attacks.

Amgen and Eli Lilly have been pursuing similar drugs and conducting large clinical trials, raising questions about the entire enterprise. The Novartis study lasted seven years and involved 8,323 people who had high Lp(a) levels and pre-existing heart disease. One in five people have elevated Lp(a) levels. Worldwide, 1.4 billion to two billion people are affected.

The complete data set will be presented in November at the annual meeting of the American Heart Association. There’s a sobering lesson here, Dr. Krumholz said. All the studies finding strong evidence of an association are not proof of a cause and effect. Cardiologists have seen this sort of failure before: In April, they were shocked when a drug that reduces inflammation failed to protect against heart disease.