The best tip for foreigners is to not do anything in Korea that you would not do at home. And as a visitor, you should treat the people and its laws with respect.

Every country has certain laws that they enforce more than others. In Korea, the use of drugs is a huge crime. If you are caught with/or under the influence of drugs you will not just be fined. You will most likely be arrested and put in jail. We cannot stress how serious this issue is. So please do not bring or look for drugs in Korea.