Pharmaceutical companies have until now failed to produce the anti-obesity drug needed by millions of Europeans and Americans.

Only two prescription drugs are currently approved for long-treatment of obesity. They are the lipase inhibitor orlistat (Xenical), the leading medicine for weight reduction, and sibutramine (Meridia). Use of both products has been limited by side effects.

A new anti-obesity drug currently awaiting decisions on approval by U.S. and European regulators, Acomplia (rimonabant), has been hailed by some doctors on the basis of clinical trial results as potentially a major breakthrough.

With alli (low-dose Xenical) approved for over-the-counter sale, it becomes the only weight-loss product in a crowded market that has been blessed by the U.S. Food and Drug Administration.

A profile of prescription anti-obesity drugs awaiting approval or already on the market follows:

Acomplia

Acomplia (rimonabant), discovered and developed by French pharmaceutical company Sanofi-Synthelabo (now Sanofi-Aventis), is a new drug currently that has shown considerable promise in trials for the treatment of obesity and related metabolic risk factors.

Acomplia works by blocking the CB1 receptor, one of two receptors found in a newly described physiological system called the Endocannabinoid System, believed to play a critical role in the regulation of food intake and energy expenditure.

The receptors are present on the surfaces of many cells throughout the body, including fat cells -- which are involved in lipid and glucose metabolism -- and those in the hypothalamus, the brain region that is thought to determine appetite.

Cannabinoids, chemical compounds produced by your body, latch on to the CB1 receptors, which are overactive in overweight and obese individuals, sending out a signal that prompts people to eat more. 

Acomplia works by selectively targeting and blocking the CB1 receptors, helping normalize the over-activation of the EC system and making hunger or cigarette pangs more manageable.

This blocking of signals that control cravings appears to result in weight loss, improvement of cardiovascular/ metabolic risk factors in overweight/obese patients and appears to reduce tobacco dependence without post cessation weight gain in people who smoke.

Meridia

Approved by the U.S. FDA in 1997, Meridia^® (sibutramine) is thought to decrease your appetiite by inhibiting the reuptake of serotonin, norepinephrine, and dopamine in the brain.

By doing this, Meridia helps increase the levels of these hormones that control appetite and hunger. It was the first selective serotonin reuptake inhibitor (SSRI) to be used specifically for weight loss, although other drugs in the same class (Prozac, Paxil) have been used as anti-depressants.

Sibutramine (which is marketed as Reductil in Europe) is indicated for obese patients with a body mass index of 30 or more, or for patients with a BMI of 27 or more who also have high blood pressure, high cholesterol, or diabetes.

The drug causes a small increase in average blood pressure, and causes a higher increase in some patients.

Phentermine

First approved almost half a century ago in 1959 as an appetite suppressant, phentermine is sold under a variety of brand names including Ionamin and Adipex-P as well as under its generic name.

Phentermine is a sympathomimetic amine, which is similar to an amphetamine. It is also known as an "anorectic" or "anorexigenic" drug. It stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure, and it suppresses your appetite.

During the 1990s, Phentermine was commonly used with another medication called Fenfluramine hydrochloride in a combination referred to as Fen-phen.

In 1997, this combination became the center of a storm, and fenfluramine and dexfenfluramine were taken off the market for possibly causing heart valve damage.

Because of possible adverse effects and contraindications for use, Phentermine is generally used to treat obesity only for a brief period of time.