Peter A Goldsbury
I looked up Web MD and saw that Coumadin is a brand name for warfarin. The Web MD page mentioned a few other blood thinning drugs, including Pradaxa, which I have been taking for several years. I have to take other, similar, drugs besides Pradaxa, but have suffered no ill effects in aikido training. My own doctor does not like warfarin because of the possible side effects and the constant monitoring required.
I have seen some excessive bruising (in one training partner) but that was eventually taken care of by modified dose. It was expected that the dose would eventually be modified so no great surprise.
I guess the take home message is keep your doctor informed.