HI, thanks for using healthcare magic Once you are still producing and releasing breast milk, you would still be using up calories to make the milk and there should still weight loss though may not be as much milk production. The more your baby feeds (particularly if exclusive) or the more milk that is produced, the faster the weight loss. If you breast feed and use a healthy diet not associated gain, then you would lose weight. In terms of the medication, the half life of phentermine is 25 hours. The half life of a medication is the time it takes for the amount of medication in the body to decrease by half. It takes 3 to 5 half lives for a medication to be mostly eliminated. This means it would take approximately 75 hours (just over three days) to 125 hours (approximately 5 days). I hope this helps