Optifast is considered to be the benchmark of liquid diets. Since the mid 1970's, it's treated over 1,000,000 obese patients, and has been proven effective in over 80 clinical studies. To participate in Optifast, you must see a participating doctor and have a physical exam - plus get regular monitoring throughout the plan. This is to make sure you are healthy enough for the rapid weight loss that is expected, and that you will be monitored for any risk factors that you may have. You also receive support, counseling, and lifestyle education. You'll also receive an activity/exercise plan that is tailored just for you.
The diet is an 18 week plan. The first 12 weeks are purely liquid. It's followed by a 6 week plan that incorporates regular food back in to the daily routine. During the liquid portion of the plan, you'll have your choise of mixes for shakes and soups, or ready-to-drink cans of meal replacement shakes. The meal replacements are balanced with vitamins and minerals, so you should not have to add any additional supplements to your meal plan.
This diet is effective, but expensive depending on your eating habits prior to the diet. Expect the initial workup to cost around $500-$600, plus the purchase price of meal replacements which amount to around $100 a week. It is expensive, but subtract what you normally spend between meals, snacks, fast food and restaurants and the difference probably isn't very much.
According to Optifast, you should expect to lose 2-4 pounds per week, and 50% of participants keep 'enough weight off to improve health long term'.