Uruguay's decision to legalise the production, sale and consumption of marijuana violates international law, the UN drugs watchdog says.
The International Narcotics Control Board (INCB) warned that the move would endanger young people and "contribute to the earlier onset of addiction".
The new law will allow registered Uruguayans over 18 to buy up to 40g (1.4oz) of the drug a month.
The government hopes it will help tackle drug cartels.
INCB chief Raymond Yans said he was "surprised" that the government in Montevideo had "knowingly decided to break the universally agreed and internationally endorsed treaty".