The International Narcotics Control Board says Uruguay has violated a 1961 global drug-control treaty with its decision to legalize the sale of marijuana.
"The decision of the Uruguayan legislature fails to consider its negative impacts on health since scientific studies confirm that cannabis is an addictive substance with serious consequences for people's health," said Raymond Yans, the group's president. "In particular, the use and abuse of cannabis by young people can seriously affect their development."
On Tuesday, Uruguay became the first nation in the world to fully legalize the sale of marijuana when the Senate voted 16 to 3 to approve the plan.
Under the new law, residents will be allowed to cultivate up to six plants and membership clubs of up to 45 members will be allowed to collectively produce 99 plants. Marijuana will also be sold at pharmacies.
In a statement, Yans said Uruguay is violating the 1961 Single Convention on Narcotic Drugs.
Yans said he was surprised that Uruguay had "knowingly decided to break the universally agreed and internationally endorsed legal provisions of the treaty."
He said that cannabis is not only addictive but may affect some brain functions, IQ potential and academic and job performance.
Uruguay joins the states of Colorado and Washington state, which will begin selling marijuana for recreational use in 2014.
Earlier, the board tried to get the U.S. government to block plans by Washington state and Colorado to begin selling marijuana next year.
But the Justice Department said in August it would allow the states to proceed.