I used drugs in my home country, where it's legal. Can Korea still punish me?

For foreign nationals, Korea generally punishes drug offenses committed in Korea, not use abroad. (Korean citizens, by contrast, can be punished for use overseas.) The difficulty is evidence: a positive hair test does not show where the drug was taken, and investigators may argue it was in Korea. Travel history, timing, and test results must be analyzed carefully. Do not assume "I used it abroad" will end the case.