The promise of fast and easy weight loss is hard to resist. But do weight-loss supplements work? And are they safe?

Stores sell dietary supplements as health aids. People take them by mouth. Dietary supplements often have vitamins, minerals, fiber, caffeine, herbs and other plants in them.

Some of the most popular supplements claim to improve diet, boost energy, build muscle or burn fat.

Dietary supplements are not medicines. They aren't meant to prevent, treat or cure medical conditions.

Companies that make supplements must make sure they're safe. They also must make sure that their products are free of harmful substances and that they have correct labels.

The U.S. Food and Drug Administration (FDA) doesn't need to approve dietary supplements. But if the FDA learns that a supplement is unsafe, the FDA can issue warnings or ask that it be taken off the market.

The FDA also can take action against companies that use false claims or claims with no proof to sell their supplements.

You might be surprised to learn that makers of dietary supplements rarely do clinical trials. That's part of the reason why there's little scientific proof to show that weight-loss supplements work.

For instance, stores sell raspberry ketone as a weight-loss product with clinical proof. The sellers of raspberry ketone base that claim on one clinical trial.

The trial included 70 adults with obesity. All the adults in the trial participated in a diet and exercise program. The trial assigned them at random to one of two groups. One group got a pill with no active ingredient, called placebo. The other got a supplement that had raspberry ketone, caffeine, bitter orange, ginger and garlic root extract.

All 45 people who finished the trial lost weight:

• The people in the supplement group lost an average of 4.2 pounds (1.9 kilograms).
• The people in the placebo group lost an average of 0.9 pounds (0.4 kilograms).

These results seem to favor the supplement group. But it was a small trial that lasted only eight weeks. That means the results can't be used to predict real-life results of using the supplement. And a short trial like this may miss side effects that only show up with long-term use.

Also, the trial used a supplement that had more than one ingredient. So it's not possible to tell which ingredient caused the weight loss.

It would be ideal if these first results had been tested in a much longer trial. Ideally the trial would have hundreds of people that researchers watched for side effects. Results from such a trial would show more about the product safety and how well it worked.

If a product doesn't have this type of trial data, be careful about believing claims about its safety and how well it works for weight loss.

Overall, little proof exists that any dietary supplement can help with healthy, long-term weight loss.

A product isn't safe simply because it's natural. Though rare, some dietary supplements have been linked to serious problems, such as liver damage.

Supplements can have strong effects. Ephedra (ma-huang) is an herb once used for weight loss. The FDA banned it because of its link to side effects, such as high blood pressure, irregular heart rate, stroke, seizures and heart attacks.

Some weight-loss supplements have been found to have ingredients that aren't listed on the label, such as prescription medicines. These can be harmful.

Other safety issues may include having an allergy to something in the supplement and interactions with other medicines you take.

Learn all you can about any weight-loss supplement you think about trying. Check websites you can trust, such as those run by the U.S. Office of Dietary Supplements and the National Center for Complementary and Integrative Health.

Keep in mind that the key to losing weight is a whole lifestyle approach that involves eating well and moving more. Supplements are rarely the answer to safe, sustained weight loss.

Also be sure to talk with your healthcare professional before taking any supplement, especially if you have health conditions, take prescription medicines, or are pregnant or breastfeeding.