Dieting is big business, thanks to the more than 64 percent of Americans who are currently classified as overweight or obese. And while diet books and fitness programs proliferate, many people seek the seemingly easy way out by popping diet pills. But while diet drugs can play a role in successful weight loss, they are not a miracle cure for obesity.

“They are not substitutes for changing your behavior,” says Gary Foster, Ph.D., director of the Center of Obesity Research and Education at Temple University in Philadelphia. “Diet drugs are ancillary treatments to be used in addition to diet and exercise.”

These drugs also aren’t intended for people looking to drop just a few pounds. Alli, the newly approved over-the-counter version of Xenical, is recommended for those with a body mass index of 25 or higher (the classification for overweight is 25 to 29.9). Prescription medications are designed for those with a BMI of 27 or higher if there are other risk factors (such as diabetes); 30 or over (the classification for obese) without other risk factors. And of course, so-called natural options also abound. Clinical evidence may be scant, but that hasn’t stopped supplements like hoodia and bitter orange extract from attracting a cult-like following among some dieters.

Here, how the options stack up.

Alli

  • How it works: This drug helps people lose weight by blocking the absorption of fat. Take the pill before a meal, and you will save yourself 25 percent of the calories from fat that the meal contains. (The stronger prescription version, Xenical, blocks 30 percent.)
  • How successful it is: As the drug’s advertisements declare, it “only works if you work.” But combined with a healthy diet and exercise program, Alli can help dieters lose 50 percent more weight than with diet and exercise alone. Experts caution that since it has to be taken before each meal in order to block fat absorption, it’s easy to be less than diligent and sit down to a double cheeseburger without first popping the pill.
  • Potential health risks: “Because it works on the gut, not the brain, it has fewer risks than other medications,” says Foster. It’s not entering the bloodstream or traveling to the liver or kidneys. One concern, however, is that while Alli blocks the absorption of fat, it’s also blocking the absorption of fat-soluble vitamins A, D, E, and K. A daily multivitamin should be included in your regimen to make up for any deficiency.
  • Nasty side effects: Well all that non-absorbed fat has to go somewhere. And as users of Alli and Xenical will find out, if you eat more than 15 grams of fat in one sitting, you may have an unfortunate accident—in the form of uncontrollable leakage and oily bowel movements.

Meridia

  • How it works: This drug works to change the chemicals in the brain so that they relay different signals to your belly. Typically, Meridia is described as an appetite suppressant, but Foster says it’s actually more of a “satiety enhancer.” “It won’t leave you with no desire to eat, but it will help you feel full on less food,” he says.
  • How successful it is: In clinical trials, about 60 percent of those who took the drug for a year lost about 5 percent of their body weight.
  • Potential health risks: The biggest problem is a risk of increased blood pressure. Although only 2 percent of users will experience this side effect, the drug is not recommended for people who have uncontrolled high blood pressure (and those who do use it are advised to get their blood pressure checked regularly).
  • Nasty side effects: Because it’s a stimulant, like all appetite suppressants, you might feel a bit nervous or jittery when taking Meridia, and it can cause sleep problems.

Bitter orange extract

  • How it works: The active ingredient in bitter orange extract is called synephrine, a stimulant that constricts the blood vessels, increases metabolic rate and heart rate. All that stimulant activity (and it’s often combined with caffeine in weight loss supplements) boosts its claim to help suppress the appetite.
  • How successful it is: Although anecdotal evidence abounds (such as testimonials on weight loss and bodybuilding Web sites), there is little science to back up bitter orange’s claims. And since products that contain the ingredient aren’t regulated by the U.S. Food and Drug Administration, companies aren’t required to have proof of success—or safety.
  • Potential health risks: The chemical structure of synephrine is very similar to that of ephedra (or ephedrine), a popular weight loss stimulant that was banned by the FDA in 2004 after reports of heart attacks and strokes. And in a 2005 Mayo Clinic report, synephrine was also linked to potential for causing strokes.
  • Nasty side effects: Racing heart rate, jitters and trouble sleeping could be a result of using this stimulant—especially if it’s combined with caffeine or you drink a few cups of coffee in addition to taking it.

Hoodia

  • How it works: No one knows for sure because the plant extract, a native of South Africa, has been subjected to very little scientific study. Some preliminary evidence suggests that it works on the brain’s hypothalamus, the area that controls appetite and satiety.
  • How successful it is: Again, that depends on who you ask. Most doctors and weight loss experts are skeptical about hoodia’s effectiveness. But manufacturers and users of the products claim dramatic reductions in caloric intake and pounds lost within weeks of using the supplement.
  • Potential health risks: It supposedly imparts none of the jittery feelings—and consequent increases in heart rate and blood pressure—of other appetite suppressants. But since these products (and their claims) are not regulated by the FDA, one potential risk is that you have no idea what you’re really getting. Products may contain more or less of the ingredient than the label claims—or even none at all!
  • Nasty side effects: None have been reported so far.

Conjugated Linoleic Acid

  • How it works: Supposedly, this compound has properties that help build muscle while also reducing body fat.
  • How successful it is: While a few studies (many on animals) have been done on CLA, the results are still inconclusive. Some have shown moderate weight loss; others have shown no difference between taking CLA and taking a placebo.
  • Potential health risks: Some researchers point to a possible increased risk of diabetes because there is some evidence that CLA could cause insulin resistance.
  • Nasty side effects: Users have reported gastrointestinal troubles, such as indigestion and diarrhea.