Hello, Thanks for letting us know your health concern. The most likely cause of your abnormal menstrual pattern seems to be due to the rapid weight loss which occurs as a side effect of use of Phentermine. Sudden weight change can lead to hormone imbalance and hence breakthrough/withdrawal bleeding due to abnormal endometrial formation. To get over this, make sure that the weight loss is not rapid and also all nutritional requirements are met during the process of treatment. In addition, you have to take proper contraceptive precaution. In the current scenario, you need styptics to arrest the bleeding. Please check with your health care provider if you need any hormones to correct the disturbance. Hope this helps.