(CBS) — Multivitamins don't work and may actually be harmful. That's the opinion of a Harvard doctor and it's causing quite a controversy.

Dr. Harvey Simon used to take multivitamins but not anymore.

"For healthy adults I can say with a good deal of confidence that they're not effective,” he said.

Writing in the Harvard Men's Health Watch, he points to research showing multivitamins “don't protect against heart disease or cancer."

And studies suggest high amounts of some vitamins may actually raise the risk of certain cancers.

"So if we say a multivitamin doesn't provide benefit and we think there's even a possibility that it could do harm, why take it?” Dr. Simon said.

For millions of Americans, including Michael Ash, the news is a hard pill to swallow.

"Usually when I get up in the morning first thing I do is take my vitamin,” he said.

Dr. Simon argues healthy adults only need a small amount of vitamins that come from foods. A bowl of cereal, he says, provides almost a full daily dose.

But many experts disagree, including Weill Cornell's Dr. Richard Rivlin.

He cites earlier studies showing multivitamins are beneficial.

"The balance of evidence really does favor the use of multivitamin supplements,” Dr. Rivlin said.

The experts do agree on some things. Vitamins are vital for pregnant women and anyone with a deficiency, including seniors.

But when it comes to healthy adults the debate continues.