Yes, you could.
You would need a written script from the doctor.
It would need to express that the sauna is a medical need.
If it has any personal use (anyone else uses it) you would need to prorate the usage.
The expense would be deducted as a medical expense on Schedule A, Itemized Deductions.
Please be aware that if you sell the sauna, you may need to claim a taxable gain.
If you deduct the cost of the sauna, and later sell it, the basis is the 2% floor that you were not able to deduct.
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