Americans spend nearly $24 billion dollars on dietary supplements each year, about half what they pay for prescription medications.

Yet many people are surprised to learn that dietary supplements, including vitamin, mineral and herbal products, are not tested or approved by the U.S. Food and Drug Administration (FDA) the way that prescription and over-the-counter medications are.

In fact, the FDA has limited regulatory authority over supplements and little manpower to back up the laws they can enforce. So the agency typically only gets involved when they’ve been made aware of serious problems.

That means there are thousands of potentially dangerous, contaminated or just plain ineffective, dietary supplements on the market that could find their way to your medicine cabinet.  

Thankfully, serious safety problems appear to be relatively rare.

But even large, reputable vitamin companies can run into quality issues.

“One out of every four supplements, or 25 percent, do have some type of quality problem,” says Tod Cooperman, MD, president of ConsumerLab.com, an independent laboratory in White Plains, N.Y., that has evaluated more than 2,000 supplements representing 300 brands.

The most common problem that ConsumerLab finds in its testing is a product with too little of the promised ingredient.

“You may find none of that ingredient, or just a small amount,” Dr. Cooperman says.

Contamination with lead, pesticides or other heavy metals is the next most common issue.

“People often ask can you tell us the name of a good manufacturer or a good brand. And I say you really can’t just make a general statement like that because they are getting their ingredients from all over [the world], so they may make a great fish oil, but their turmeric, or whatever, may be contaminated,” Dr. Cooperman says.

Some supplements, particularly those that claim to aid weight loss and erectile dysfunction and products that promise to help you build muscle, have even been spiked with prescription medications.

So how can you protect yourself and make sure you’re getting what you’re paying for? Experts recommend the following five ways to be more supplement savvy.

Watch for Overblown Claims

With the exception of certain nutrient deficiencies, it’s against the law for manufacturers to claim that their products treat, cure or prevent any diseases or conditions. But the FDA doesn’t pre-approve product labels, so many deceptive claims make it onto the market where they may dupe consumers.

In recent Senate testimony, the Federal Trade Commission, which enforces the ban on false claims, said that in the past two years it has filed or settled more than 30 claims against supplement manufacturers for misleading claims related to everything from colds to cancer.