2. Can the UN punish countries that legalize marijuana? Only to a small degree. The UN International Narcotics Control Board is the keeper of the drug treaties and regularly chastises governments that violate their commitments. This can be embarrassing in international diplomatic circles, but no nation has ever collapsed due to embarrassment.
Because the International Narcotics Control Board has power over the production and transport of the legal medical supply of drugs it could in theory punish a country that legalized marijuana by imposing punitive controls on pain medications. But the international humanitarian outcry over such an action would be enormous. Further, the countries that produce the most opiate painkillers are not easy to push around (e.g., Australia, India, The United States). It is thus safe to assume that crimping the medical supply of drugs is a trigger that the Narcotics Control Board is not going to pull.