Per IRS Publication 17, Tax Guide for Individuals, page 159:
If you are covered under social security (or if you are a government employee who paid Medicare tax), you are enrolled in Medicare A. The payroll tax paid for Medicare A isn't a medical expense.
If you aren't covered under social security (or weren't a government employee who paid Medicare tax), you can voluntarily enroll in Medicare A. In this situation, you can include the premiums you paid for Medicare A as a medical expense.
Medicare B is supplemental medical insurance. Premiums you pay for Medicare B are a medical expense. Check the information you received from the Social Security Administration to find out your premium.
Medicare D is a voluntary prescription drug insurance program for persons with Medicare A or B. You can include as a medical expense premiums you pay for Medicare D.
See IRS Publication 17 Your Federal Income Tax (For Individuals) for more information.
To enter or review information from Form SSA-1099, including Medicare Parts B and D premiums:
To enter Medicare Parts B and D premiums not reported on Form SSA-1099: