I've been to SK as a tourist. They don't drug test you if you're a SK citizen, but they will drug test you if you're a foreigner applying for a SK work visa. They will require you to submit medical forms from your doctor to their embassy which includes a drug test, and they may test you again on arrival. They can and do arrest people for having detectable amounts of drugs in their bodies, but I don't know which drugs are tolerable and which aren't, or the acceptable levels. AFAIK if you're a foreigner and they catch you, it's instant deportation and a lifetime ban.
When my friend went to SK to teach English they did a urine test on arrival and made him pay for it, but his employer reimbursed him. This was last year. I don't know if the policy has gotten more strict.
As for NK, you can visit there if you enter through China, but the requirements are strict and honestly that country is so fucked up I would not want any of my tourist dollars going toward their government. You are accompanied by a government propaganda shill the entire time and they control your every movement. I had an opportunity to go there when I was in Beijing but decided against it. I felt that the young woman who was excited to do it was completely delusional and had no idea what she was in for. I later met an illegal immigrant in China who was from NK and he had learned enough Mandarin to convey to me the horrors of his country. Bullet dodged.