... lost 58 lbs toward my goal of 100 lbs. My heart rate has been staying up above 100. Should I get off the Phentermine? It has really helped me not be hungry and I am being monitored by a physician. I am on 500 calories a day and 50g of carbs. My body is in Ketosis and the weight loss period is 12 weeks. I did the 1st 12 weeks fine. Lost 30 lbs. On maintenance for 2 months, then started 12 week program again. My heart rate is a concern to me and when I exercise on treadmill my heart rate goes to 135 pretty quick.