I'm a Korean-American dual citizen and I used cannabis legally in California — can Korea really prosecute me for that now that I'm in Seoul?

The investigator was not using a scare tactic. Korean criminal law applies to Korean nationals for offenses committed anywhere in the world, and Korean courts have convicted Korean nationals for cannabis use in countries where it was legal. A dual national is a Korean national for that purpose; your US citizenship is not a defense and is not, in a Korean court, a relevant fact. That is the hard part. The remaining questions — whether the police can prove use, whether you must give them the means to, and how a first-offense case of this kind is disposed of — are where your position is better than it feels.

What this is under Korean law

Article 3 of the Criminal Act (형법, hyeongbeop) provides that the Act applies to Korean nationals who commit crimes outside Korea. The Narcotics Control Act (마약류관리법, mayangnyu gwalli-beop) is applied through that provision, so a Korean national's cannabis use in California is, in Korean law, an offense under Article 61 as if it had occurred in Seoul. The legality of the act where it was done is not a defense; the Supreme Court has held that a Korean national is bound by Korean narcotics law abroad. This is why the government warns Korean travelers to Canada, and why the cases you found exist.

Dual nationality does not change it. Under the Nationality Act, a person who holds Korean nationality is treated as a Korean national in Korea regardless of other nationalities, and the Criminal Act's personal jurisdiction attaches to that status. If you had renounced Korean nationality before the use, the analysis would differ; you have not.

Proof, and the test

A prosecution needs proof of use within the period Korean law can reach — and hair analysis can detect cannabis use for months. A request for a hair or urine sample at a police interview is a voluntary investigation (임의수사, imui susa); you may decline. The police may then seek a warrant for a sample, and a court may grant one where the friend's statement gives reasonable suspicion. Refusing is not an offense and does not create an adverse inference in law; in practice it prolongs the case and shifts it onto the warrant track.

Whether to consent is a decision for counsel on your facts — including when you last used, what a test would show, and whether the friend's statement is the only evidence. Do not decide it in the interview room, and do not give a further statement about "only in the US" without advice; that sentence is an admission of use by a Korean national, and it is already on the record once.

How these cases are disposed of

A first-offense use case for a Korean national, with a positive test and an admission, is ordinarily resolved at the prosecutor's desk: a conditional suspension of indictment (기소유예, giso yuye) with a treatment or education condition, or a summary fine. Where use was frequent and long-term — "regularly" in California — prosecutors are more likely to indict, and a suspended sentence follows. The file's account of frequency matters; "regularly" and "a few times" are different files.

Your situation as a dual national

You are not a foreign national for immigration purposes; there is no visa to lose, no departure suspension of the foreign-national kind, and no deportation question. What you have instead is a Korean criminal record if convicted, which follows you as a Korean citizen, and, if you are of military-service age, the interaction between a criminal disposition and your service obligations — a matter to raise with counsel promptly. Your US side is unaffected by a Korean disposition unless you must disclose foreign convictions on a US application.

What to do now, and what not to do

Do not use, in any country, until this is over. Do not contact the friend. Retain counsel before any further interview or test. Do not volunteer frequency or dates; answer questions your lawyer has prepared you for. Consider a voluntary drug education program now.

From a former police investigator's perspective

Investigators handling dual nationals know the personal-jurisdiction rule better than the suspects do, and they know that "it was legal there" is what every such suspect says first — and that it is an admission. What they are deciding is whether the file supports use recent enough to charge and whether the person is a habitual user or a visitor from a different legal culture who has stopped. A dual national who stops, gets advice, and does not argue with the statute is the second kind.

Contact us before you give a sample or another statement — text consultation or +82-2-522-3959.

— MEGA X Law Firm Korea, Criminal Defense Team