It does sound like a side effect from coumadin which is a blood thinner but I'd be willing to bet your father is diabetic. Those blisters you describe could be diabetic bullas, also known as bullous eruption of diabetes mellitus. It's a cutaneous condition characterized by a noninflammatory, spontaneous, painless blister, often in acral locations, seen in diabetic patients. He needs to be seen ASAP. It is possible that that the coumadin has forced water from the surrounding and no where to go but through the skin. The acral area is generally a non haired area of the body such as palms, feet and under the nails. The consition is treatable if caught early which is sounds like you have. The doctor may prescribe a water bill if he's not on one already and either insulin or metformin if he is in fact diabetic. Diabetic bullas generally show up around people in their sixties or older.
