Calories consumed in food are used by the body as fuel. The body’s use of calories as its source of energy is called thermogenesis. Literally, thermogenesis means “the production of heat” because when people burn calories, heat is produced. The body needs a constant supply of fuel to maintain normal functions that people don’t usually think about, like breathing.

Certain drugs cause the body to burn more calories as it performs its normal daily functions. By burning more calories, people lose weight even though they are eating the same amount. These drugs increase the body’s fuel requirements, ideally to a point where the body needs more calories than the person takes in from food. When this happens, the body begins breaking down its fat stores as fuel. As these fat stores are used, people lose weight.

Side effects associated with this type of diet pill often occur because, in causing the body to burn more calories, the pills speed up many body processes — possibly to a level that becomes dangerous. Examples are increased blood pressure and heart rate, which can lead to heart problems over time. Other side effects include nervousness and insomnia.

Many diet pills that work by increasing the number of calories burned have been developed. Most of these contain one or more of the three ingredients: caffeine, phenylpropanolamine (PPA), and ephedrine. In the United States, PPA and ephedrine have been withdrawn from the market because they produce serious side effects, including heart attack and stroke.

Reference:

1. Diet Pills (Drugs: the Straight Facts) by Debra Henn and Deborah DeEugenio (Hardcover – Oct 2004)