To be successful, this market must be kept small, contained and profitable.
That means fixing just the right amount of active ingredient, THC, in seeds that can be cloned, identified and controlled. And with fewer than 200,000 habitual smokers in the country of 3.3 million, just 10 hectares (25 acres) of pot plants could supply the entire market.
"I'll be nervous tomorrow," Calzada confessed to The Associated Press as he watched the debate from the Senate gallery.
Mujica acknowledged that his government isn't totally prepared for this plan, which poses a direct challenge to the global drug war, which an increasing number of national leaders and experts are calling an open failure. But he said the country has to try.
"Einstein said that there's nothing more absurd than trying to change the results by always repeating the same formula. That's why we want to try other methods," he said in an interview published in Wednesday's La Republica newspaper. "We know we've started down a road where there's no university to tell us what to do. But we have to try, because there's no blind man worse than the one who doesn't want to see."
Mujica, 78, has said he wants the market to officially open by mid-2014.
Polls have showed two-thirds of Uruguayans are opposed to the plan. Many fear the government won't be able to control the market once it's legalized. Mujica, who says he never smoked pot and considers it a "plague," said he hopes they'll come around to his point of view.
"We want to free future generations from this plague. This should be a national cause. We hope the people will understand and help."
The law requires growers, sellers and users to be licensed and registered so that the government can enforce limits such as the 40 grams a month any adult will be able to buy at pharmacies, or the six marijuana plants license-holders can grow at home.
Uruguay was advised by marijuana legalization groups funded by billionaire currency speculator and philanthropist George Soros and his Open Society Foundation and Drug Policy Alliance. Mujica met with Soros and billionaire David Rockefeller, who also favors legalization, in New York in September to explain what he called his "experiment."
"I would say to Mr. Soros, to Mr. Rockefeller, and to the president of the republic that you don't experiment with the Uruguayans. We are not guinea pigs," Colorado Party Sen. Pedro Bordaberry declared in the Senate debate.
But Uruguay's plan is now a global test case, and what happens next will be closely examined as politicians elsewhere decide what to do.
"Last year, Colorado and Washington; this year, Uruguay, and next year, Oregon and hopefully more states as well," said Ethan Nadelmann, executive director of the Drug Policy Alliance.
Associated Press writer Michael Warren in Buenos Aires, Argentina, contributed to this report.