"Sellers of these supplements might claim that their products help you lose weight by blocking the absorption of fat or carbohydrates, curbing your appetite, or speeding up your metabolism. But there’s little scientific evidence that weight-loss supplements work," as the National Institutes of Health says.
At best, they'll do little for your waistline, and at worst, they'll do serious harm.
Unlike prescription drug makers, diet pill makers are largely unregulated: they don't have to prove their cocktails of herbs and minerals actually work, and they alone are responsible for making sure their supplements are safe. However, the US Food and Drug Administration (FDA) sometimes steps in to take supplements off the market, or force manufacturers to change their labeling, if supplement makers are lying about the effectiveness of their product and there's no evidence to support their claims, or if a product is deemed unsafe.
For example, the FDA took Ephedra off the market in 2004. The plant product can suppress appetite and increase calorie burn in the body, stimulating the nervous system and leading to weight loss, but it's also deadly dangerous and can lead to nausea, vomiting, anxiety, mood changes, high blood pressure, abnormal heartbeats, strokes, seizures, heart attacks, and death. Diet pills can also interact with other drugs you may be taking in strange ways, so it's important to check with your doctor before you start popping one.