Drug makers would love nothing better than to develop effective anti-obesity medications, but it is proving very difficult to manipulate the body's powerful weight-control systems...

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Drug makers would love nothing better than to develop effective anti-obesity medications, but it is proving very difficult to manipulate the body’s powerful weight-control systems without serious side effects.

Only three FDA-approved drugs are on the market:

Meridia: One of two drugs approved for long-term use, Meridia is an amphetamine-like compound that boosts brain levels of two neurotransmitters — serotonin and noradrenaline. It lessens hunger pangs and creates a feeling of fullness, resulting in a 5 to 10 percent weight loss.

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But it also increases blood pressure and is the subject of a class-action lawsuit and an FDA investigation. Worldwide, 34 people have died while taking the drug, several from heart problems; manufacturer Abbott Laboratories says the death rate among the 8.5 million people who have taken Meridia is much lower than normal for an obese population.

Xenical: Also approved for long-term use, it reduces fat absorption by blocking certain enzymes. People who take Xenical while dieting lose 2 to 3 percent more weight than through diet alone, but many gain it back. Severe diarrhea is a common side effect.

Phentermine: The phen portion of the infamous combination drug fen-phen, or Redux. “Fen” was banned in 1997 after reports of death and heart problems. Phentermine, which suppresses appetite, is approved only for short-term use.

Many other drugs are in the pipeline, including:

Cannabinoid receptor inhibitors: These compounds block the brain receptors responsible for “the munchies” associated with marijuana use. The hope is that the drugs will suppress the urge to eat.

Axokine: Originally developed to treat Lou Gehrig’s disease, this synthetic hormone activates brain cells that decrease appetite. In continuing trials, obese patients lost 4 percent of their body weight and kept it off for up to a year.

Melanocortin 4 receptors activators: When triggered by weight-regulating hormones, Melanocortin 4 receptor brain centers inhibit appetite. Obese people develop resistance to the hormones.

By targeting the brain receptors directly, scientists hope to overcome that problem.

Human trials will start next year, but animal studies have turned up a troublesome side effect: persistent and unpredictable erections in males.