On "The Early Show", CBS News Medical Correspondent Dr. Jennifer Ashton said this is a widespread and dangerous issue.
Ashton explained, "(The problem) when you are talking about dietary supplements, even if they are 'natural products,' is that the FDA does not have to police their safety before they come to market. They are only responsible for ascertaining their safety after they are on the shelf."
The FDA, Ashton said, is concerned about three popular categories of supplements that are available both on the Internet, and in some cases in retail stores, for weight loss, sexual enhancement and bodybuilding.
In the realm of weight loss, Ashton said there's concern over a substance called sibutramine that is in the drug Meridia, which was recently pulled from the market.
She said, "The concern is not only liver problems, but stroke and potentially heart problems and heart attacks. Talking about weight loss, 'natural' does not always mean safe."
With bodybuilding products, Ashton said, "They sometimes promote on the packaging it is a legal alternative to anabolic steroids, which we know have very dangerous side effects, liver problems and can really have serious hormonal consequences."
She added, "The other issue talking about anabolic steroids, sometimes the package will give a warning saying you may test positive on drug testing. You have to realize that if it works to build muscle and can give you a positive drug test there, that there are substances in there that can work as anabolic steroids do, which is potentially very dangerous."
Sexual enhancement drugs can also have issues with illegal and dangerous substances, Ashton said.
"The FDA is issuing a warning for people to stop taking categories of these supplements because the concern here is they contain a drug very similar to the prescription medication in Viagra, which can affect blood pressure," she said.
Ashton said the four items the FDA warns people not take are Vigor-25, Duro Extend Capsules for Men, Magic Power Coffee and Man Up Now.
She said, "They're available on the internet and in stores. If people are taking them, they should stop immediately."
Ashton advised talking with your doctor about any products you are taking, even if they claim to be "natural."
"There are very good books -- you can get one called the PDR (for Herbal Medicines)," she said. "You can actually read and research these products. You cannot assume things are natural or safe. We have to remember cocaine comes from a natural plant, and that's obviously not safe."