South African scientists have been testing Hoodia since 1996 when they discovered the plant contained a previously unknown molecule that replicates the effect glucose has on nerve cells in the brain  fooling the body into thinking it is full, even when it is not.  Hoodia's appetite suppressing molecule is said to be almost 10,000 times stronger than glucose. A U.S. pharmaceutical company spent over 400 million dollars researching this molecule.
Hoodia Gordonii has been used for generations by South African tribesmen to starve off hunger and thirst during long hunting expeditions. Hoodia is difficult to grow and requires just the right amount of water, sun light, and temperature. It takes five years for Hoodia to grow to full maturity.
To test the Hoodia, morbidly obese people from Leicester, England, were kept in a place as close to prison as it gets.  All the volunteers could do was read papers and watch television and eat. Half the group was given Hoodia Gordonii and half was given  a placebo.  At the end of 15 days, the group on Hoodia had reduced their food intake by 1000 calories a day. Given the average daily diet is around 2200 calories, this was a remarkable success! Another clinical study performed in 18 overweight but healthy males provided statistical evidence that the Hoodia Gordonii extract reduced the average daily calorie intake and reduced body-fat over the 2 week clinical trial period.
 

In addition to eliminating hunger and suppressing the appetite Hoodia also seems to boost energy levels and counteract depression, some folks even report aphrodisiac-like qualities. The efficacy of Hoodia Gordonii has been established with a profound weight loss and a good emergent safety profile.