Hello, 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. https://www.sciencedirect.com/science/article/pii/S0944711306000407 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.