It depends, but if the CPAP is prescribed it is more than likely a deductible expense. (See more detail below). It is important to note that in order to deduct medical expenses, you must itemize your expenses, plus the medical expenses are subject to the 7.5% of AGI rule. (Only the amount of medical expenses that exceed 7.5% of your adjusted gross income are allowable to be included in the itemized expense deduction.)
Here is a description of the expenses that are deductible as well as what is not:
The IRS allows you to deduct preventative care, treatment, surgeries and dental and vision care as qualifying medical expenses. You can also deduct visits to psychologists and psychiatrists. Prescription medications and appliances such as glasses, contacts, false teeth and hearing aids are also deductible.
The IRS also lets you deduct the expenses that you pay to travel for medical care such as mileage on your car, bus fare and parking fees.