When a doctor tells the public "Don't take your vitamins", it's a big deal. And it's an even bigger deal if that doctor is Paul Offit, the chief of the infectious diseases division at Children's Hospital of Philadelphia and well known as a champion of preventing disease through vaccines.

The message "Take your vaccines and don't take your vitamins" makes sense for many people in light of the scientific evidence balancing the risks with the benefits.

Offit's advice on vitamins was the headline of a story in the New York Times Sunday Review and one of the messages in his new book Do You Believe in Magic?: The Sense and Nonsense of Alternative Medicine.

He said in an interview last week that he used to swallow vitamins as a sort of mindless matter of course, but in researching the book, he found that the scientific evidence weighs against their benefit for healthy people and some respectable studies show that high doses of vitamin A, beta carotine and vitamin E in supplement form can increase risk of cancer.

New research is casting doubt on the well-known theory that "free radicals" cause cancer and aging, and by deactivating free radicals, antioxidants extend life and protect us from cancer. The reality is more complicated — and newer research suggests that free radical may help us by killing germs and cancer cells.

That doesn't mean supplements are all bad. There's good evidence that a lack of folic acid increases risk of a birth defect called spina bifida and that fortifying flour with folic acid caused a vast reduction in its incidence. There may be people who benefit from vitamin D supplements – though it's not technically a vitamin but a hormone that's produced in our skins when exposed to sunlight.

The supplements of most concern to Offit are the antioxidants, since several studies show they may increase risk of cancer and heart failure in some populations. If studies like those showed a similar risk for a drug, the FDA would step in to force it to be sold with a warning label, or taken off the market. But a law enacted in 1976 tied the hands of the FDA when it came to regulating supplements.

Offit generated shock waves when he stated that antioxidant vitamins could be as bad as Vioxx – an anti-inflammatory that was taken off the market for increasing risk of heart attacks and strokes. People still think of Vioxx as pure evil and vitamins as benign.

The drug did offer relief for some people with debilitating arthritis. The tragedy with Vioxx was that the public wasn't warned about the risks because FDA didn't know about them right away. The tragedy with vitamins, however, is that FDA can't warn the public.

Offit said he came across another surprise in researching other types of alternative medicine – which is the broader subject of his book. Sometimes, he found, lying can be beneficial. People who think they are getting an effective treatment for a painful or uncomfortable condition often really do feel better. A false belief in the effectiveness of a treatment can cause a real physiological reaction. Which raises a big question: "Is it ethical to deceive?"

In some cases that may be a legitimate question, but only if there's no potential at all for harm or a risk patients would forego real treatments.