How Do Diet Pills Work

In the struggle to lose weight, many people turn to weight loss medications. Diet pills help with weight loss by stimulating metabolism, interfering with the absorption of fat from the digestive tract or increasing the levels of neurotransmitters that suppress appetite. Diet pills should only be used in obese people when their physicians believe that the health risks from being overweight are greater than the potential side effects from the medication.

Appetite Suppressants

Kinds of drugs that suppress the appetite include sympathomimetics and SNRIs (serotonin-norepinephrine reuptake inhibitors). They increase the levels of neurotransmitters that suppress the appetite.

Sympathomimetics

Amphetamines, phentermine (Fastin and Adipex) and diethylpropion (Tenuate) are sympathomemetic drugs. Sympathomimetics stimulate the hypothalamus (the portion of the brain that regulates the nervous system) to release the neurotransmitters like norepinephrine, dopamine and serotonin. These neurotransmitters stimulate the "fight or flight" response, which increases metabolism and reduces the appetite. The most common side effects of sympathomimetics include insomnia and increased blood pressure and heart rate. They should only be used under the direct supervision of a physician.

Serotonin-Norepinephrine Reuptake Inhibitors

Serotonin norepinephrine reuptake inhibitors, like the sympathomimetics, are prescription drugs that increase the levels of serotonin and norephinephrine in the central nervous system. SNRIs prevent the nerve synapses from reabsorbing these neurotransmitters, rather than stimulating their release from the hypothalamus. As of 2009, Meridia (sibutramine) was the only SNRI approved for weight loss. It is in the same class as antidepressants like Effexor and Cymbalta, but Meridia does not appear to help with depression. Side effects of SNRIs are insomnia, elevated blood pressure and increased heart rate. SNRIs should not be stopped suddenly. Those who wish to stop taking SNRIs should do so under the guidance of their physician.

Orlistat

Orlistat prevents the absorption of dietary fats from the digestive tract. Orlistat is available by prescription as Xenical, or over-the-counter in a lower strength known as Alli. Orlistat must be combined with a low-fat diet to be effective and to reduce side effects. Because it can interfere with the absorption of fat-soluble vitamins, people taking orlistat should take a multivitamin while taking orlistat. Those taking orlistat often experience diarrhea and stomach upset. In some cases, the diarrhea is so severe that bowel control is lost. These side effects go away when the drug is stopped, but for many people they are severe enough to make them stop taking the drug.

Over-the-Counter Products

The manufacture of over-the-counter diet aids is not well regulated, and their safety is questionable. Over-the-counter diet pills often contain thermogenics like caffeine, green tea and bitter orange. Thermogenics speed up the body's metabolism, which causes the body to burn more calories. They also increase blood pressure and heart rate. Ephedra (also known as ma huang) was a widely used thermogenic that was banned by the FDA in 2004 because of its link to several sudden deaths. Bitter orange is structurally similar to ephedra and can increase blood pressure and heart rate--particularly when used in combination with caffeine.

Chromium, pyruvate, hydroxycitric acid, 7-keto-DHEA and conjugated linoleic acid are other products found in over-the-counter weight loss drugs. There are no reliable scientific studies to back up the claims made by the manufacturers of these products. Because these products are marketed as dietary supplements and not drugs, their safety has not been thoroughly tested, and little is known about possible drug interactions and adverse effects.