While the machine, humidifier, and complete mask kits require a doctor's order, you can purchase many over-the-counter: Can You Buy a CPAP Machine Without a Prescription?

A doctor uses sleep study data to determine the specific air pressure required to keep your airway open. Using the wrong pressure can lead to ineffective treatment, headaches, or discomfort.

Your prescription often specifies the type of machine (CPAP, APAP, or BiPAP) and mask style best suited for your diagnosis.