Yes, you can include your Medigap insurance premiums in medical expenses on your federal tax return. However, only medical expenses in excess of 7.5% of your Adjusted Gross Income (AGI) are deductible. (Enter the full amount that you paid in TurboTax. TurboTax will subtract 7.5% of your AGI.) In addition, medical expenses are an itemized deduction. They will not reduce your tax or increase your refund unless your total itemized deductions are more than your standard deduction.
Also, you cannot claim itemized deductions in TurboTax Free Edition. You would have to upgrade to Deluxe.
