Opinion polls suggest most Uruguayans are against the pot plan, but Mujica told The Associated Press in an interview Friday that his government has got to try it.
"I want to rescue society's right to experiment. If it didn't exist, we would be condemned to paralysis, stuck in a photo that never changes a bit. There is no other way to be able to advance," the president said.
Mujica and his governing Broad Front coalition ministers signed the rules behind closed doors, passing up the opportunity for a public ceremony on an issue that has dominated public discussion in Uruguay recently.
The president's efforts may be celebrated on T-shirts emblazoned with a green cannabis leaf and the phrase "Mujicannabis," but Mujica himself played down the signing, opting instead to eat pizza with some friends at a downtown restaurant.
With Mujica's signature, the regulations are now fully in effect, deputy presidential secretary Diego Canepa told the AP.
That means Uruguayan citizens and legal residents 18 or older can register to obtain licenses giving them the right to cultivate up to six marijuana plants per household and harvest 480 grams a year, or join a marijuana growing club with between 15 and 45 members and no more than 99 plants.
By putting his government at the center of a legal marijuana industry, Mujica hopes to keep otherwise law-abiding citizens away from organized crime and treat addiction as a public health challenge rather than a law enforcement threat.
That's easier said than done: Police on Monday were investigating the slaying of a 24-year-old man preparing to harvest marijuana from six plants he kept in his home. According to the newspaper El Pais, a witness told police that Darwin Porley was shot on the sidewalk Sunday night after two men demanded all his plants, saying he was intruding on their turf. Officials said it wasn't known if the plants were for personal use or for sale.
Under the law, pot plants like Porley's will be legal to possess for personal consumption once a grower is registered.
In two weeks, the government plans to take bids and choose a handful of growers to provide marijuana to the government for sale. Licensed buyers will be able to purchase up to 10 grams a week or 40 grams a month from a network of pharmacies, at a cost that will begin at about 90 cents a gram but be adjusted to compete with illegal weed.
The rules establish that the Uruguayan state will sell five varieties of pot, with no more than 15 percent THC, the substance that makes people high. Buyers will use a fingerprint identification system so the government can track their purchases without them having to identify themselves in stores.
By no means will Uruguay become a pot smoker's paradise, Mujica told the AP. If anything, the government is headed in the other direction: The Senate approved a law Tuesday that would prohibit the public display of tobacco cigarettes in stores. The measure now goes to the House.
"There is no imaginary solution that is sold by the bottle or cigarette," Mujica said. "That's just escapism. And in life there are many things that are worth trying as an adventure, but not imaginary adventures that end up enslaving a life."
Uruguay's public health system is "certainly not ready" for the consequences of the new system and will have to adjust along the way, the president added.
"Faced with a challenge, the challenge motivates us, pushes us and we'll confront it along the way," he said. "This idea of having everything planned out in advance is something beautiful for people who manufacture novels."