Uruguay is struggling to roll out the commercial production and sale of marijuana and its experiment could be dropped or watered down if an opposition candidate wins this month's presidential election.

The South American country is the world's first to permit the cultivation, distribution and use of marijuana, aiming to wrest control of the trade from drug gangs while at the same time regulating and even taxing its consumption.

The reform is being followed closely across Latin America where the legalization or decriminalization of some narcotics is increasingly viewed as a better way to end the violence spawned by drug trafficking than the US-led "war on drugs".

But first Uruguay needs to work out how to ensure criminal gangs do not finance producers, how to regulate the supply and quality of locally produced marijuana, and how to satisfy neighbouring states that legally grown dope will not be sold illegally on their streets.

The government has missed its own deadlines in implementing the changes.