A pre-clinical trial evaluated the effects CBD, THC and CBN had on blood clotting. Researches found that THC and CBN significantly hindered the clotting response.
However, because these effects have yet to be studied in humans, and because of the possible grave consequences which may ensue if blood clotting were not properly modified, I do not recommend using cannabis as a blood thinner when treating serious conditions.
As Dr. Kim points out, the citation above references a study done on artifically obese rats. In a normal human individual there is little or no consistently observed, clinically significant, direct effect of cannabis on clotting. Two important caveats. One, if you are taking a medication that is metabolized using the same processes the body uses to metabolize cannabis then the level of that medication could go a bit higher when using cannabis than otherwise. If that medication is an anticoagulant like Coumadin or Xarelto there could be a (small) effect on clotting. The other caveat is that most people don’t know if they have some coagulation problems unless their clotting system is stressed, which is not a good time to be finding it out. So anyone who expects to be having significant surgery should consider reducing cannabis use in the week before surgery.