I was on Optifast 7 years ago and did lose alot of weight. Unfortunately I gained it all back, that is a different story. I do remember that I had to drink alot of water and even then had weeks with no loss or alittle gain. It is hard to believe that I was taking under 500 calories a day, product has changed since then, and not losing weight. I use to say "how can that be". I remember getting really frustrated in class also.
Optifast is a great product and it made me believe I could get down to my high school weight, which I am doing again, just not with Optifast. It is to expensive for one thing and there are a few other reasons. I will not get into them on here, but if you go to my blog you can read about them.
If you are not losing weight fast on Optifast or if you are just on Optifast you need to go read these 2 articles on my blog. One is by me and the other one I found online. Hopefully the two articles will help you lose the weight that you want to lose.