But in an interview with The Daily Telegraph conducted amid piles of agricultural tomes, in the dimly lit front room of his bungalow, where he has lived for 30 years, he expressed his surprise at the furore.
"It's targeting the battle against drug trafficking," he said. "It's not a law supporting addiction. It's a way of battling the black-market economy."
Waxing lyrical, even on such a contentious issue, he continued: "Every addiction is a scourge, except for love."
Under the radical new law not only are citizens and private businesses allowed to grow, buy and sell cannabis, but the government will also enter into the business itself - cultivating and harvesting the crop, distributing and selling it from authorised outlets and taxing the profits.
The legislation is much more sweeping than in the famously liberal Netherlands, where cultivation remains banned, or the new arrangements in the American states of Colorado and Washington, where there is no direct government involvement.
Mr Mujica and his allies argue that the new policy offers a ground-breaking alternative to what they call the failures of the "war on drugs" championed by the US in Latin America, which has killed tens of thousands in producer and transit countries while comparatively little is done in the consumer nations of Europe and the US.
The law also aims to deliver a major blow to the narco-cartels trafficking hard drugs by removing one of their biggest money-earners. "If you want to change a reality, it's foolish to continue doing the same thing, instead of changing something, even if it hasn't been successful," Mr Mujica said.
File photo / APN
Under the new law, Uruguayans will be able to buy a limited amount of cannabis, expected to be capped at around 1.4oz (40 grams) a month, from pharmacies or from Post Office-type one-stop shops, which are normally used for paying bills or changing money.
Users will have to register on a government-held database and those who make excessive requests will be referred to health authorities for treatment. The database is also intended to ensure that only Uruguayan residents can purchase the product in an effort to reduce the risks of drug tourism.
In a traditionally conservative Roman Catholic country, opinion polls indicate that many Uruguayans dislike the innovation introduced by the Broad Front Left-wing coalition, either because they suspect drug use will grow or disapprove of a government role in the drug trade. Mr Mujica is, however, undeterred, saying that his opponents are simply "scared" by such dramatic change, while acknowledging that the new law could pave the way for the decriminalisation of other drugs.
He has allies in the region: other Latin American leaders including Otto Perez Molina, the Guatemalan president, have also spoken out against the war on drugs, while Enrique Pena Nieto, the president of Mexico, has ended his predecessor's relentless military struggle with the narco-traffickers.
Amid more cautious views in Europe, last month Norman Baker, the Liberal Democrat Home Office minister, held an hour-long video conference with Uruguayan officials, discussing the public health and security implications of the new law.
There are meanwhile practical challenges for implementing the policy, not least that Uruguay does not produce enough cannabis for supply to meet demand. Broad Front leaders, including Lucia Topolansky, a senator who is also the first lady, suggested that Canadian medical marijuana producers could temporarily fill the gap.
That was news to Canada, where a health ministry spokesman said there were "no plans" to export marijuana to Uruguay, or anywhere else.
