Research into antioxidant supplements such as vitamins A, C and E suggests the pills are more likely to do harm than ward off cancer and heart disease.
Antioxidants are thought to prevent and limit damage to our body's cells caused by free radicals as we age.
While it's difficult to overdose on antioxidants in fruits and vegetables, it's easier when taking a pill with high concentrations, said Prof. Jim Kehrer of the pharmacy department at the University of Alberta in Edmonton.
Advertisers have put forth the idea that a little is good, more is better and a lot is great but that isn't really correct, said Kehrer, who has been researching the effects of free radicals since the 1970s.
"You get to a point, and sometimes early, that the high doses become hazardous," Kehrer said in an interview with CBC News.
"The good thing about antioxidants is that you have to take absolutely massive doses to get overt toxicity. But that doesn't mean that there can't be subtle effects building up over many periods of years if you take more than you should be getting."
Nutrient antioxidants include vitamins such as A, C and E. Minerals such as selenium and manganese act as antioxidants indirectly in enzymes.
Natural phytochemical antioxidants are present in foods. There are two major classes. One is the carotenoids that include beta-carotene, lycopene, lutein and zeaxanthin. The other is the group of polyphenols such as anthocyanins that give colour to fruits and vegetables.
Antioxidants can also be classified as being water soluble (ascorbates, glutathione, urate, bilirubin, polyphenols that include a number of compounds present in fruits and vegetables and give them their distinctive and attractive colours).
The other major group is the lipid soluble ones: vitamins A and E, alpha and beta carotene, lycopene, other carotenoids, ubiquinol.
In general, it's thought that water soluble vitamins do not pose a major health problem since they can be excreted from the body in the urine. However, large doses can put stress on the kidneys and can also upset the balance in the immune system.
Lipid soluble antioxidants, like other lipid soluble compounds, can be a problem since they are not excreted efficiently and tend to accumulate in the body to unsafe levels.
Source: Prof. Venket Rao
Taking antioxidants in excess can overwhelm free radicals that cells use to talk to one another and function properly, which Kehrer said is probably part of the reason why taking too many isn't as good as taking the right amount.
Eating a mixture of fruits and vegetables has a safety factor built in to prevent exposure to massive doses while getting beta carotene from carrots, lycopene from tomatoes and polyphenols from grapes, for example, said Venket Rao, a professor emeritus in the nutritional sciences department at the University of Toronto.
Rao said a megadose is five to 10 times the Estimated Average Requirement or EAR, which varies by age and sex. A single capsule of vitamin E for example wouldn't be a megadose, but a man aged 25 to 50 who takes multiple capsules that approach 1,000 mg/day and above could be getting a megadose, he explained.
Studies in Canada, the U.S., Australia, Italy, Finland and elsewhere have shown that giving megadoses of antioxidants like vitamin E to people with lung cancer and cardiovascular diseases actually promotes oxidative stress instead of countering it, Rao said.
Oxidative stress results when there is an imbalance between free radicals and the body's ability to detoxify them or repair the damage.
"That's the paradox," Rao said. "We have evidence that antioxidants are good for you, but we also have well-controlled clinical studies showing that at megadoses they could be harmful to you. So what do you believe in?"
For example, a 2009 review of randomized trials on antioxidants published by the Cochrane Library concluded that "Antioxidant supplements need to be considered medicinal products and should undergo sufﬁcient evaluation before marketing."
High levels of vitamin A can cause severe liver damage, Kehrer said. He suggested that people need a better grasp of good nutritional intake and not try to overdo selected chemicals.
At the produce section of a grocery store in Toronto, shopper Bora Skenderi called the marketing a fad.
"You hear about all the super foods," Skenderi said. "If you lead a healthy lifestyle, you get all the antioxidants."
"I think that fresh is best," said shopper Patrice Brennan. "The only time that I really go for the antioxidants is in the winter when I can't really buy fresh."