It is common for people on blood thinner (anticoagulant) medication such as warfarin to have more bruising than usual and for these bruises to take longer to heal. This can vary from person to person.
So it really depends on how the medication is affecting you as well as how hard a spanking you are getting (how much damage).
I would just suggest that you start of light with the spanking and see how things go. If you have been on the medication for a while you probably already have an idea of what sort of hit your body can take before getting a bruise and how long it takes to heal e.g. Hitting your shin on a coffee table etc...
