If half your social security plus all your other income is less than $25,000 (or $32,000 if married filing jointly) then your social security is not taxable and your other income is less than the standard deduction, so it is also not taxable.
There will be an exception if you are married, file a separate return from your spouse, and your spouse itemizes deduction on their tax return.
Participating in a medical study is not "work" and the payment is not considered "compensation." The payer may issue a 1099-MISC, but you don't have to file a return if the other facts are as you stated.
However, if the payer issues a 1099-NEC, the IRS may assume that you owe self-employment tax, even though you don't owe income tax. There are a couple ways you can handle that, if you got a 1099-NEC instead of a 1099-MISC.