Can I deduct the cost of a treatment for Prostate Cancer that the FDA hasn't approved?

[Mike9241]

Examples of Medical and Dental Payments You Can't Deduct

• Illegal operations or drugs.
• Imported drugs not approved by
the U.S. Food and Drug Administration
(FDA). This includes foreign-made versions
of U.S.-approved drugs manufactured
without FDA approval.

however, it's best to check IRS PUB 502 which contains greater detail of medical expenses 

https://www.irs.gov/forms-pubs/about-publication-502 

[TomD8]

IRS definition of medical expenses you CAN deduct (subject to IRS limits):

"Medical expenses are the costs of diagnosis, cure, mitigation, treatment, or prevention of disease, and for the purpose of affecting any part or function of the body. These expenses include payments for legal medical services rendered by physicians, surgeons, dentists, and other medical practitioners. They include the costs of equipment, supplies, and diagnostic devices needed for these purposes.

Medical care expenses must be primarily to alleviate or prevent a physical or mental disability or illness. They don't include expenses that are merely beneficial to general health, such as vitamins or a vacation."

[Opus 17]

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@Mike9241 wrote:

Examples of Medical and Dental Payments You Can't Deduct

• Illegal operations or drugs.
• Imported drugs not approved by
the U.S. Food and Drug Administration
(FDA). This includes foreign-made versions
of U.S.-approved drugs manufactured
without FDA approval.

Let me amend this answer slightly. 

Doctors are allowed to use FDA-approved drugs for off-label purposes if, in their medical judgement, the drug would be beneficial.   If Drug A is FDA-approved to treat disease X, and your doctor wants to try it on your cancer, your out of pocket expenses would be deductible even though prostate cancer is not one of the FDA-approved uses.  However, if drug A is not FDA-approved for any condition, then it is not an allowable deductible medical expense on your tax return.