Several anti obesity medications had weight loss amounts of 5.9, 4.7, and 4.2kgs.

Average long term weight loss for diet 2.0 or 7.1kgs(only 55% of people responded so huge selective bias could be present)

I think the gap is nonexistent or much closer than you think. It looks like pills are pretty close to as effective at causing weight loss as changing your diet.

http://en.wikipedia.org/wiki/Management_of_obesity#Medicatio... http://ajcn.nutrition.org/content/74/5/579.short