It's like stepping into a time machine. The internet and its resources are available, yet no amount of reasoning or information gets through to a lot of people. Marijuana is just "evil" or only "people who commit/will commit crimes" use it.
I don't necessarily think the attitude here should be just like in the US, but at least let people use it medically, and get rid of the disproportionate stigma.
As @laureig mentioned, one can’t even admit trying marijuana in polite/semi-polite company (of other Koreans). With foreigners it’s a little different. Generally it only gets discussed if you’re closer with them.
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At the same time, alcohol [and alcoholism] is not only totally fine, it's encouraged! Sometimes it's even mandatory to go drinking with your coworkers after work, due to social pressure from peers and bosses. I have spoken with a few people like this.
But going to clubs in 홍대 Hongdae/강남 Gangnam and drinking until 9am or drinking at a dozen bars in Golden Gai until 6am is totally fine. Drinking until you puke on the street is totally fine. (Admittedly I did do this once in 세종시 Sejong-City, Korea)