Vitamin E provides little protection against heart attacks, strokes and cancer - despite millions of people believing it does, a major study shows.
The vitamin did not protect against heart disease or cancer
Some previous trials had suggested it might be beneficial, prompting one in 10 US women to take it as a supplement.
But a Women's Health Study of 40,000 women aged over 45, in the Journal of the American Medical Association, should settle the debate, say experts.
The women were followed from 1992 to 2004 to see what effect vitamin E had.
The authors of the study, published in the Journal of the American Medical Association, say there is enough evidence now to tell healthy women that taking vitamin E will not protect them against the two big killers - cardiovascular disease and cancer.
The women in the study were followed for a decade to see what effect vitamin E or aspirin taken every other day had on their risk of strokes and heart attacks and cancer.
Some of the women received real tablets, while others received dummy pills.
Vitamin E (600 IU) did nothing to reduce cardiovascular disease risk. There were 482 heart attacks or strokes, some fatal, in the group of women who received vitamin E compared to 517 in the placebo group.
In comparison, aspirin (100mg) reduced the risk of stroke overall in the women studied and the risk of both stroke and heart attack in those aged 65 and older.
When it came to cancer prevention, neither vitamin E nor aspirin appeared to work. But there was some suggestion that aspirin might prevent lung cancer.
The 10mg tablets appeared to reduce the risk of lung cancer by a fifth and death from lung cancer by nearly a third.
The researchers said this might be worth investigating further, as well as the possibility that higher doses might prevent other cancers.
The Harvard Medical School team said their trial was "the longest of any completed to date" to look at such trends and should, therefore, "be sufficient to detect long-term effects".
Dr Elizabeth Nabel, director of the National, Heart, Lung and Blood Institute, which funded the study, said: "We can now say that despite their initial promise, vitamin E supplements do not prevent heart attack and stroke.
"Instead, women should focus on well proven means of heart disease prevention, including leading a healthy lifestyle."
The researchers suggested that the fact that people who take vitamin supplements and eat food rich in vitamins tend to be generally more healthy anyway might explain why past work found a benefit with vitamin E.
More aspirin research
Current US and UK guidelines do not recommend vitamin E for cardiovascular disease or cancer prevention.
High doses of vitamin E and aspirin can be harmful.
The British Heart Foundation said its own research had shown taking vitamin E has no significant effect on cardiovascular health.
A spokeswoman advised: "Stopping smoking, lowering blood pressure and cholesterol levels, are the best known ways to prevent coronary heart disease.
"People can also reduce their risk by increasing the amount of physical activity they take, reducing the amount of saturated fat and salt in their diet and eating at least five portions of fruit and vegetables per day.
"Taking vitamin supplements should not be seen as a replacement for taking these measures."
Ed Yong of Cancer Research UK said: "Studies have shown that aspirin could protect against certain types of cancer.
"A key difference could be in the doses tested. Cancer Research UK is currently funding large studies to investigate the potential benefits of aspirin in cancer prevention."