Two other studies found fish oil didn't work for an irregular heartbeat condition called atrial fibrillation, even though it is thought to help certain people with heart disease or high levels of fats called triglycerides in their blood.
The bottom line: Dietary supplements have varied effects and whether one is right for you may depend on your personal health profile, diet and lifestyle.
"Many people take vitamin supplements as a crutch," said study leader Dr. Howard Sesso of Brigham and Women's Hospital in Boston. "They're no substitute for a heart-healthy diet, exercising, not smoking, keeping your weight down," especially for lowering heart risks.
The studies were presented Monday at an American Heart Association conference in Los Angeles and the vitamin research and one fish oil study were published in the Journal of the American Medical Association.
Multivitamins are America's favorite dietary supplement - about one-third of adults take them. Yet no government agency recommends their routine use for preventing chronic diseases, and few studies have tested them to see if they can. A leading preventive medicine task force even recommends against beta-carotene supplements, alone or with other vitamins, to prevent cancer or heart disease because some studies have found them harmful. And vitamin K can affect bleeding and interfere with some commonly used heart drugs.
Sesso's study involved nearly 15,000 healthy male doctors given monthly packets of Centrum Silver or fake multivitamins. After about 11 years, there were no differences between the groups in heart attacks, strokes, chest pain, heart failure or heart-related deaths.
Side effects were fairly similar except for more rashes among vitamin users. The National Institutes of Health paid for most of the study. Pfizer Inc. supplied the pills and other companies supplied the packaging.
The same study a few weeks ago found that multivitamins cut the chance of developing cancer by 8 percent - a modest amount and less than what can be achieved from a good diet, exercise and not smoking.
Multivitamins also may have different results in women or people less healthy than those in this study - only 4 percent smoked, for example.
The fish oil studies tested prescription-strength omega-3 capsules from several companies in two different groups of people for preventing atrial fibrillation, a fluttering, irregular heartbeat.
One from South America aimed to prevent recurrent episodes in 600 participants who already had the condition. The other sought to prevent it from developing in 1,500 people from the U.S., Italy and Argentina having various types of heart surgery, such as valve replacement. About one third of heart surgery patients develop atrial fibrillation as a complication.
Both studies found fish oil ineffective.