How quickly does phentermine lose its efficacy?

I have read that Phentermine should be taken up to 12 weeks at a time. There are different doses out there. My doctor is starting me with 15mg for 90 days and if everything goes well he will increase the dose to 30mg.
To help with your hunger, try drinking about 8oz of water. Usually after I drink some water, my stomach doesn't even want to think about food. Also, try chewing gum. It helps with the dry mouth and it gives your mouth something to do. Sometimes it's good to go over, about 200 cals, from your calorie limit. Sometimes eating more will help the weight come off too.