I believe you're referring to Switzerland. It isn't legal, but addicts can get free heroin (or methadone) administered by doctors. It cuts the bottom out of the dealer's markets, prevents petty crime by desperate addicts, avoids OD deaths, and makes sure the government has tabs on how bad the problem is. In 20 years Zürich has gone from 'having a heroin problem' to just a few old-timers who occasionally relapse, with almost no new addicts.