PHOENIX -- People take vitamins to stay healthy, but a CBS 5 investigation found they may be causing illness instead.

Vitamins can be dangerous not only if too many are taken, but also because of certain ingredients.

"It's got polyethylene glycol, which is antifreeze," Dr. Sam Benjamin said while examining a bottle of simple calcium tablets.

Benjamin said there's a lot more in a multi-vitamin that may be bad for the consumer's health.

"If you don't take the exactly right multivitamin, you're doing yourself potentially harm," Benjamin said. "The concept that these things are benign is not true."

In fact, too much of certain vitamins through diet and supplements can actually lead to disease.

An overdose of vitamin A has been linked to osteoporosis in women.

Elevated levels of iron have been associated with heart problems.

And while folic acid is important for women who are pregnant or are trying become pregnant, it can be dangerous for others.

"Folic acid may be associated with increasing colon cancer in those who already have pre-malignant states," Benjamin said.

Too much vitamin E has been associated with thickening of the artery walls, while large amounts of vitamin C have been linked to plaque in the arteries.

Benjamin said people should check their multi-vitamins to make sure they are made in the U.S. and independently tested.

The amount of each vitamin a person needs depends on several factors including age, sex and diet. For more information, click here.