Phentermine (Adipex-P, Lomaira) is a prescription medicine used to lessen appetite. It can help weight loss by making you less hungry. Or it can help you feel full longer. Phentermine also is offered combined with topiramate for weight loss (Qsymia). The combined drug is approved for long-term use. Adipex-P should be used only for a short-term period. It should be used as part of a complete weight reduction program that includes dietary changes, exercise, and other behavioral changes. Detailed Adipex-P dosage information What happens if I miss a dose? Take the medicine as soon as you can, but skip the missed dose if it is late in the day.