Forskolin comes from the Indian lant Coleus Forskohlii, which is a native Indian plant that has long been used in Hindu and Ayurvedic medicine to treat various illnesses including cardiovascular, respiratory, gastrointestinal, and central nervous system disorders. The root portion of Coleus Forskohlii contains the active compound forskolin. Coleus Forskohlii is a member of the mint family and grows in regions of India, Nepal, Sri Lanka, and Thailand. The plant is rich in alkaloids, and grows one to two feet tall.
Forskolin was discovered in 1974, and it is the primary constituent of clinical interest, extracted from the root of Coleus Forskohlii. Aside from its herbal uses for treating various conditions, Coleus Forskohlii has applications for weight loss, where studies in animals and humans have found some clinical evidence to support its benefits for aiding weight loss.
Forskolin is found to act on adenylate cyclase which is an enzyme that activates cAMP - which promotes the breakdown of stored fats in fat cells of animals and humans . It regulates the body's thermogenic response to food, increases the body's basal metabolic rate, and increases utilization of body fat. It may also release fatty acids from adipose tissue, which results in increased thermogenesis, loss of body fat, and theoretically increased lean body mass. Because Forskolin increases cAMP accumulation, it stimulates lipolysis (breakdown of fat). 
Because the Coleus Forskohlii extract, forskolin, works to increase cAMP accumulation, it stimulates the breakdown of fats stored in fat cells, which should enhance fat loss and weight loss. Because of this, Coleus Forskohlii is used as an ingredient in many fat burners and diet pills.
Coleus Forskohlii Weight Loss Benefits
Studies on both lab animals and human subjects have found weight loss benefits of supplementing Coleus Forskohlii; however, there is only a small number of studies done on Coleus Forskohlii and weight loss benefits. Nonetheless, the results of these studies are encouraging, and we summarized a few of these studies below.
In a study involving lab rats, Coleus Forskohlii was administered to the rats to determine its weight loss effects. The rats were given either a placebo in their diet, or a 50g/kg dose of Coleus Forskohlii extract. Measurements of body weight, adipose tissue, and cell diameter were taken. At the end of trials, the study found that consuming Coleus Forskohlii reduced body weight, food intake, and fat accumulation in the rats, suggesting that Coleus Forskohlii can be useful to treat obesity. 
Another published study from the Journal of the International Society of Sports Nutrition also found weight loss benefits of Coleus Forskohlii extract. This was a double blind, randomized study involving 23 female human subjects, who were supplemented with Forslean (250mg of 10% Coleus Forskohlii extract) twice a day for 12 weeks. Body weight was taken at 0, 4, 8, and 12 weeks, and blood samples and dietary records were taken a 0 and 12 weeks. Coleus Forskohlii side effects was taken on a weekly basis.
Their results found that supplementing Coleus Forskohlii extract helped mitigate weight gain in the female subjects; however they did not find a significant difference in fat mass, fat free mass, or body fat in the female subjects. The females subjects who took Coleus Forskohlii reported less fatigue and hunger. Their results suggests that Coleus Forskohlii does not appear to promote weight loss, but may help to prevent weight gain with no side effects. 
In a study done at the University of Kansas, the effects of Coleus Forskohlii extract was tested on body composition, testosterone, metabolic rate, and blood pressure in overweight and obese male subjects. Thirty overweight male subjects were randomly assigned to either a placebo or a Coleus Forskohlii extract for 12 weeks.
Their results found that Coleus Forskohlii extract resulted in favorable changes in body composition and significantly decreased body fat percentage and fat mass in the male subjects. They also found that the Coleus Forskohlii group had a significant increase of lean body mass, bone mass, and the free testosterone levels of the Coleus Forskohlii group increased significantly as well. While free testosterone levels increased, they did not find a significant difference in the serum total testosterone levels between the two groups. They concluded that oral supplementation of Coleus Forskohlii extract favorably altered body composition, reducing body fat, increasing lean mass and bone mass, and increasing serum free testosterone levels. 
As you can see from these few studies done on Coleus Forskohlii extract, it has fat loss benefits, and also helps to increase lean mass, improving body composition. However, looking at these studies, it would appear that Coleus Forskohlii extract is more effective for decreasing fat and increasing lean mass for men than women. But more studies are needed in this area to determine if this is in fact the case.
Coleus Forskohlii Side Effects
Coleus Forskohlii is generally safe with little to no side effects. The studies done on Coleus Forskohlii extract reported no adverse side effects. However, these are just a small number of studies. Coleus Forskohlii may lower blood pressure - anyone with low or high blood pressure, or cardiovascular diseases should be cautious about using it. If you have these conditions, it's best to consult your physician before using Forskolin.
Learn the true method of long term, healthy weight loss that will lead to permanent weight loss results.
1. Forskolin as an activator of cyclic AMP accumulation and lipolysis in rat adipocytes.
Litosch I, Hudson TH, Mills I, Li SY, Fain JN: Mol Pharmacol 1982, 22:109-15.
2. Effects of Coleus Forskohlii Supplementation on Body Composition and Hematological Profiles in Mildly Overweight Women
Shonteh Henderson, Bahrat Magu, Chris Rasmussen, Stacey Lancaster, Chad Kerksick, Penny Smith, Charlie Melton, Patty Cowan, Mike Greenwood, Conrad Earnest, Anthony Almada, Pervis Milnor, Terri Magrans, Rodney Bowden, Song Ounpraseuth, Ashli Thomas and Richard B Kreider
Exercise & Sport Nutrition Laboratory, Baylor University, Waco, TX
3. Yakugaku Zasshi. 2005 May;125(5):449-53.
Effects of Coleus forskohlii on fat storage in ovariectomized rats.
Han LK, Morimoto C, Yu RH, Okuda H.
4. Obes Res. 2005 Aug;13(8):1335-43.
Body composition and hormonal adaptations associated with forskolin consumption in overweight and obese men.
Godard MP, Johnson BA, Richmond SR.