Grapefruit recipe substitute to avoid medicine interaction?
Two ideas come to mind: 


Artificial grapefruit flavor. Clearly not the most desirable ingredient, but they make it and you can buy it online if you look for it. 
Hoppy beer (IPAs) for aroma. Certain hop varieties have a strong grapefruit aroma. You can seek out West Coast-style IPAs for this (Lagunitas IPA or Lil' Sumpin and Ballast Point Sculpin are some that come to mind). Or homebrew your own beer with hops like Cascade, Amarillo, Centennial, Citra, and there are several others. You can google for grapefruit hops.


Otherwise, you might try meyer lemon. It's sweeter than a lemon, but still has some sourness, though not the same as grapefruit.