You can claim the money paid for medicine for your in-law if you are able to claim your in-law as a dependent on your tax return.
According to the IRS Publication 502, you can include medical expenses you paid for an individual that would have been your dependent except that:
1. He or she received gross income of $4,200 or more in 2019;
2. He or she filed a joint return for 2019; or
3. You, or your spouse if filing jointly, could be claimed
If eligible to claim the expenses, then enter the cost of the prescriptions in the Medical Expenses section of your return.
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