The largest study to test daily multivitamin use finds they modestly lowered the risk for cancer in middle-aged and older men but did not seem to affect the chances of developing heart disease.
The studies did not include women but other research under way now does.
Two other studies found fish oil didn't work for an irregular heartbeat condition called atrial fibrillation. Fish oil is believed to help certain people with heart disease or high levels of fats called triglycerides in their blood.
Doctors say that dietary supplements have different effects and are no substitute for a healthy diet and lifestyle.
The studies were discussed Monday at an American Heart Association conference in Los Angeles.