The IRS only addresses gym membership in the context of weight loss for a specific condition:
You can include in medical expenses amounts you pay to lose weight if it is a treatment for a specific disease diagnosed by a physician (such as obesity, hypertension, or heart disease). This includes fees you pay for membership in a weight reduction group as well as fees for attendance at periodic meetings. You can't include membership dues in a gym, health club, or spa as medical expenses, but you can include separate fees charged there for weight loss activities.
See Weight-Loss Program in the IRS' Publication 502, Medical and Dental Expenses.
That said, a deduction wouldn't necessarily be restricted to weight-loss issues. Please see MichaelMc's comments above, as they're right on target. Membership fees can be deducted only if a doctor has diagnosed you with a specific medical condition, or a specific physical or mental defect or illness, and you have written documentation of this diagnosis. The health club facilities must be used to treat the specific condition, defect, or illness as recommended by your doctor.
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