Why in the world would someone have a heart attack and blood clot at age 23 ? There is a serious underly ing health condition in most people with a lot of unexplained serious illnesses and without knowing it, cannot give much risk assessents. Certainly if the marijuana makes one just lie about all day it is risky for the development of blood clots. Coumadin/warfarin are risky drugs which most anything could interact with either directly or indirectly by changes in diet and activity. While there are not direct effects of marijuana on clotting and bleeding, there certainly could be several indirect effects that would change clotting while on blood thinners and this would be a reason t either not do a lot of things, or to have close monitoring of their effects.