Evidence suggests that phentermine can help improve sleep quality and quantity in patients with sleep problems like obstructive sleep apnea, a sleep disorder characterized by involuntary cessation of breathing for brief periods of time (apnea) during sleep:
In obese subjects with moderate to severe obstructive sleep apnea who were unable or unwilling to comply with standard treatment, administration of phentermine 15 mg plus extended-release topiramate 92 mg significantly reduced symptoms.
In obese adults, phentermine treatment reduced the incidence of apnea.