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June 1 (Reuters) - Weight-loss pills that suppress the appetite and block fat absorption are a significant part of the multibillion-dollar dieting industry that critics say should be better regulated.

Here are some facts on slimming drugs:

WHICH ARE THE MOST POPULAR?:

-- Statistics are scarce as many drugs are unregulated.

-- Two prescription drugs, orlistat, marketed by Roche under the brand Xenical, and sibutramine, marketed by Abbott as Reductil in the UK, and as Meridia in the U.S., are approved for use by the Food and Drug Administration

-- Both are only prescribed for patients with a Body Mass Index (BMI) of 30 or more, or a BMI of 27 or more if they have other weight-related health problems

-- Various non-prescription drugs and herbal supplements are available from the Internet, chemists or private "diet clinics"

HOW DO PRESCRIPTION DIET DRUGS WORK?:

-- Some are fat blockers or "lipase inhibitors", like orlistat. They block fat-digesting lipase enzymes in the intestinal tract, so about a third of dietary fats are excreted before they are broken down into small molecules and absorbed.

-- Others, like sibutramine, are appetite supressants or "satiety enhancers". They block nerve cells that release and reabsorb serotonin, increasing serotonin levels and acting on the brain to promote the feeling of fullness.

AND OTHER DIET DRUGS?:

-- Over the counter drugs used for weight control range from diuretics, forcing the body to lose fluid; to stimulants, speeding up the heart rate and metabolism to burn fat more efficiently; and laxatives, stimulating the large bowel to empty before food and calories are absorbed via the small intestines.

-- Some abuse other drugs to lose weight. Hollywood celebrities have reportedly turned to the 'Size Zero Pill' -- a steroid-like drug called Clenbuterol, legal only for horses, which burns fat by accelerating the heart rate and increasing body temperature.

HOW EFFECTIVE ARE THEY?:

-- Generally, when taken in conjunction with a healthy diet and exercise, some are effective in the short term, experts say.

-- Many over the counter pills are unregulated, and may be useless. Pills are also sometimes taken inappropriately, reducing their likely efficacy -- a 1998 study showed as many as a quarter of U.S. weight loss pill users were not overweight, for example.

-- An International Journal of Obesity review found appetite suppressants were almost two times as effective as fat blockers: Orlistat gave an average 2.9 percent loss of body weight when taken for a year, and sibutramine resulted in a 4.6 percent loss over the same period.

WHAT ARE THE RISKS?:

-- All drugs carry health risks. Appetite suppressers have been linked with side effects like dry mouth, anorexia, insomnia and constipation and convulsions.

-- Fat blockers, which also reduce the absorption of fat-soluble vitamins A, D, E and K, and beta-carotene, may be associated with increased defecation or fecal incontinence.

DO MOST PEOPLE NEED TO LOSE WEIGHT?:

-- Globally, there are more than 1 billion overweight adults, and at least 300 million of them are clinically obese, the World Health Organisation estimates.

-- Being obese or overweight poses a major risk for chronic diseases, including type 2 diabetes, cardiovascular disease, hypertension and stroke, and certain forms of cancer.

-- The U.S. weight loss market is worth at least US$40 billion, analysts say.

Sources: Anti-Obesity Forum (http://nationalobesityforum.org.uk/content/view/101/151/), Food and Drug Administration (www.fda.gov/bbs/topics/NEWS/2007/NEW01557.html),

Phentermine (www.phentermine.com), the World Health Organisation (www.who.int/dietphysicalactivity/publications/facts/obesity/en/)

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