In order of most effective....
Based on published studies:
DRUG/ STUDY DURATION/ TOTAL Kg WEIGHT LOSS/
1) Fen-Phen/ 26 weeks/ 16.5
2) Orlistat/ 12 weeks/ 4.74
3) Sibutramine/ 24 weeks/ 7.6
4) Fluoxetine/ 52 weeks/ 13.9
5) Phentermine/ 24 weeks/ 5.05
6) Dexfenfluramine/ 52 weeks/ 9.82
A small note to consider......
7-Keto also fares well in comparison to a number of prescription weight loss drugs. The daily supplement of 200mg of 7-Keto resulted in an average weight loss of 0.36 kg per week, falling in the middle range compared to weight loss drugs, which range from a high of 0.63kg per week for Fen-Phen and a low of 0.19kg per week for Dexfenfluramine. On the safety front, 7-Keto comes out on top, as there were no adverse side effects reported throughout the study, unlike several of the weight loss drugs.
Hope this helps!