Fat burners (sometimes known as thermogenics) were often made with an herb called ephedra, also known as ephedrine or ma huang, which acts as a stimulant and increases metabolism. Some athletes use fat burners to lose weight or to increase energy but ephedra-based products can be one of the most dangerous supplements. Evidence has shown that it can cause heart problems, stroke, and occasionally even death.

Because athletes and others have died using this supplement, ephedra has been taken off the market. Since the ban, "ephedra-free" products have emerged, but they often contain ingredients with ephedra-like properties, including bitter orange or country mallow. Similar to ephedra, these supplements can cause high blood pressure, heart attack, stroke, and seizures.

This combination may lead to restlessness, anxiety, racing heart, irregular heart beat, and increases the chance of having a life-threatening side effect.