Uruguay's House of Representatives will hold a vote to legalize marijuana late on Wednesday.
If the bill passes both the House and Senate, it would pave the way for Uruguay to be the first country to legally regulate production and distribution of the drug.
Of the 99 members of the House voting, 50 will have to say "yes" in order for the bill to be signed into law.
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Growers, sellers and consumers, all aged 18 and above, would be licensed by a confidential registry to keep people from buying more than 40 grams a month.
Anyone carrying, growing or selling without a license would face stiff punishments, including long prison terms.
The law has been backed by President José Mujica, despite polls showing two-thirds of Uruguayans are opposed to it.
"I have never tried it in my life and I don't know what it is," Mujica told a local radio station. "[I am aware] a lot of young people have tried it."
The purpose of the bill would be to give the government legal control of the marijuana market, creating enough quality product to drive out illegal dealers and draw a line between pot smokers and those who use harder drugs.