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That may vary state by state. Here in MN, we don't get consideration for sales tax, but it might be considered part of the cost of the spa that might get deducted.
I would really be afraid to try to deduct the cost of my tub, even though my motivation for purchase was arthritis, also from Lyme. My tub is also too large to say I am the only one who benefits from it
After being hit by a car and numerous trips to the Dr. and a chiropractor, it was suggested that a hot tub would help in treatments, and so on. Because of this I used the cost of my spa as a medical expense and was allowed to deduct the entire cost off my taxes. I was told by a former IRS tax prep person that if a Dr. prescribes this, it can be counted as an uninsured medical expense and can be used as a tax deduction. However, it is very important that a medical professional recommends this as part of ongoing treatment.