Americans spend some $5 billion on multivitamins every year. Consumer Reports says most people don't need to take a multivitamin, particularly if they eat a balanced diet. But it's necessary for pregnant women and people on strict diets.

And a new survey from Consumer Reports National Research Center finds that around half of the multivitamin users polled weren't sure they were taking the right one, and they're worried about contamination.

Consumer Reports tested 21 multivitamins—regular daily multivitamins, as well as one for seniors and chewables for children. An outside lab tested for the ingredients claimed, as well as for contamination. None contained worrisome levels of heavy metals or excessive doses of any vitamin or mineral.

Consumer Reports also tested to see how well the multivitamins dissolve. That's important, so that you're able to get all of the nutrients.

Two of the most expensive multivitamins had a problem. Some samples from Rite Aid's Whole Source Mature Adult and Vitamin Shoppe's One Daily did not dissolve sufficiently.

And with the One Daily, samples from two of the three lots tested contained only 73 percent of the Vitamin A listed.

All of the other multivitamins passed Consumer Reports' tests, so you can choose by price. One of the least expensive for all three types was Equate from Walmart. And the Kirkland Signature multivitamins from Costco will save you even more.

And if you really want to cut your costs, Consumer Reports says look for multivitamin sales and buy in bulk, because many vitamins don't expire for at least a year.