According to the FTC, many of these claims suggest that you can lose weight without cutting back on calories or exercising more. To lose one pound of weight, you need to eat 3,500 fewer calories or burn 3,500 additional calories. Drugs may make this easier to do, but the calorie cutback must occur. The FTC says that no over-the-counter (OTC) drug, dietary supplement, cream, wrap, device or patch can cause you to lose weight without also cutting back on calories or exercising more. Prescription drugs can block the absorption of fat or interfere with the breakdown of carbohydrates, but these are used under a doctor's care. They may cause side effects that include diarrhea, cramping, bloating and gas.

The FTC says it is impossible to lose more than a pound a week without cutting back on calories or increasing exercise.

Prescription drugs such as Xenical block the absorption of fat, but they work only modestly. They are used under a doctor's care along with diet and exercise. Products that claim to make you lose two pounds a week would need to block 1,000 calories a day, the FTC says. Incredibly, some products claim to make you lose two pounds a DAY. They would need to block more than 7,000 calories a day, but that is several times the amount most people consume.

Your metabolism refers to the rate that your body's burns calories. Ephedra was another supposed metabolism booster, but its effect also was modest -- and that was only because it acted to suppress appetite. It also caused serious medical problems, including heart attack and hemorrhagic stroke. The Food and Drug Administration (FDA) has banned the sale of dietary supplements containing ephedra. Synthetic ephedrine is regulated as a drug by the FDA. It has not been banned and is allowed in OTC medicines used for asthma, nasal congestion and minor eye irritation. Some diets claim that certain foods like grapefruit or cabbage soup burn fat and make you lose weight. This is not true, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says. Although some foods containing caffeine may speed up your metabolism for a short time, they do not cause weight loss.

No product can bring about permanent weight loss, the FTC says. Most diets end in failure because people are unwilling to change their lifestyle for the long term, seeking instead a quick fix. Healthy (and feasible) weight loss is no more than a pound a week. Any more than that without a doctor's supervision may not be safe. No product that is worn (rings, earrings, shoe inserts) or rubbed on the skin can cause weight loss, the FTC says.

In the Atkins diet, you eat a low-carbohydrate, high-protein, high-fat diet. People like this diet because they get to eat "forbidden" high-fat foods, such as bacon cheeseburgers, steak, cheese omelets and butter. But by limiting fruits and vegetables, you may not get the vitamins, minerals and other essential nutrients you need. Over the long-term, this diet may increase your risk for heart disease and cancer, because of the emphasis on saturated fat. No long-term studies have been done to determine whether this diet is safe. Short-term studies have shown that people on this diet lose more in the first six months than with a conventional low-fat diet. At the end of a year, however, the number of pounds lost on the Atkins diet is about the same as with a conventional diet.

Over the long-term, this diet may increase your risk for heart disease and cancer. The South Beach diet rates carbohydrates by how quickly they are digested and how they affect the level of glucose in the blood. This rating system is called the glycemic index. Refined carbohydrates like white bread, white rice and pasta get a bad rating on the glycemic index scale. Unlike the Atkins diet, which bans most carbohydrates, the South Beach diet allows complex carbohydrates like brown rice and whole grains. Most of the early weight loss on this diet is in the form of water and lean muscle mass, not fat, says the NIDDK. People often get bored with high-protein/low-carb diets because they crave the carbohydrates they aren't allowed to eat. Another variation is the Zone diet. On this relatively protein-heavy plan, you consume carbohydrates, protein and fat in a ratio of 40/30/30 in specific serving sizes called "blocks." Although this diet is more moderate than others, it can be difficult or confusing to keep track of the carbohydrates, protein and fat you're eating. Nutritionists say this diet can help you lose weight because it restricts calories, but it's complicated.

These diets overwork your kidneys, and can cause dehydration, headaches and bad breath, the NIDDK says. They also can make you feel nauseous, tired, weak and dizzy. On these diets, you lose water because your kidneys try to get rid of the excess waste products of protein and fat that your body makes. A buildup of these waste products in your blood can cause your body to produce high levels of uric acid. Uric acid is a risk factor for gout, which a painful swelling of the joints, and kidney stones. This condition can be risky for pregnant women and people with diabetes.

This diet emphasizes low-fat, vegetarian eating, with a fat intake of no more than 10 percent of daily calories. It limits the amount of processed foods and encourages eating high-fiber foods. Exercise and meditation are also key elements of this diet/lifestyle. Studies have found that this diet may lower cholesterol levels modestly, as well as treat and possibly prevent heart disease. Nutritionists fault the diet, however, for its extremely low-fat approach. Few people can stay on such a diet, they say.

Low-fat or nonfat foods, particularly snack foods, may still have plenty of calories, the NIDDK says. These foods often have extra sugar or other ingredients added to make them taste better. Sugar, extra flour and thickeners all have calories, which can add up if the foods are eaten in excess. The number of calories in a food depends on the amount of carbohydrate, protein and fat it contains. Carbohydrate and protein have about 4 calories per gram, and fat has more than twice that amount, at 9 calories per gram. Good alternatives for snacking are fruits and vegetables, most of which are naturally low in fat and calories.