I checked with John Partlow of Dragon's Lair in Pueblo, Colorado. His answer was that he would not do one on someone taking warfarin. The decreased clotting ability of the blood will cause the ink to smudge. You will wind up with one of those tattoos that looks like a sailor got during World War II. One netizen wrote that she got them when her INR was below 2.0. An INR below 2.0 is essentially the same thing as taking no warfarin -- you have little or no protection from a clot -- your blood clotting ability is about the same as normal.