Yes, by lowering your calorie level drastically (1200 calories is for small petite people or sedentary people), you will be slowing your metabolism and preventing your body from getting essential nutrients provided with sufficient calories needed for every day activities . The active you are the more calories you will need. alli has accounted for your weight loss when calculating calories and activity level. If for some reason your weight stays the same after a couple of weeks(and you have not exceeded your calories and fat) then you may go down in increments of 200 calories - 1600 is your next level down.
When you have a minute read the two top post in this forum, they will help you understand the importance of reaching your calories and fat on most days.
Hope this helps,
Sylvia Melendez-Klinger, MS, RD, LDN, CPT - registered dietitian
PS: As a side note most people consume about 3,000 calories but don't even know about it. Calories add up super fast, even when you are eating healthy so enjoy your 1800 can make the best out of your plan.