MONTEVIDEO (Reuters) - Uruguayans will be able to choose from three varieties of state-sanctioned cannabis when marijuana starts being sold in pharmacies in the small South American country next year, the National Drug Board said Saturday.

Each variety will have different levels of the mind-altering compound tetrahydrocannabinol (THC) and cannabidiol (CBD), believed to soften the impacts of THC, said the board's president, Milton Romani.

"There will be three options with indications about the effects of each that point beginners toward starting with the lowest level" of THC, Romani told Reuters on the sidelines of the marijuana forum Expocannabis 2015.

Romani declined to specify the names of the three varieties.

Uruguay became the first country to legalize the cultivation and distribution of marijuana in late 2013, aiming to wrest control of the trade from gangs while regulating and taxing its consumption.

Authorities have developed traceable, genetically-distinct plants to ensure they do not leave Uruguay's borders or end up on the black market, Romani said.

The government estimates registered marijuana users will be able to buy the drug from pharmacies in mid-2016, when the country's two licensed producers start selling their first commercial load of about four tonnes.

Marijuana will be sold in Uruguay at about $1 per gram, with locals allowed to buy up to 40 grams per month.

Uruguay's pioneering marijuana law has drawn world attention at a time when policymakers are debating whether strict anti-drug laws are effective.

Last month Mexican President Enrique Pena Nieto said he would open a national debate to review the country's marijuana laws following a landmark court ruling.

Uruguay has granted marijuana-growing licenses to two companies and so far registered 3,200 people who can grow up to six plants for personal use.

Authorities are also working with companies interested in exploring the possibility of exporting medical marijuana to the United States and Europe.

But the government has struggled to meet some of its own deadlines for implementing the legislation passed two years ago.

"We want to do things right, and if takes a bit longer, so be it," Romani said. "We have to be very careful with this because any slip-up could ruin an experience that is unique in the world."

(Reporting By Malena Castaldi, Writingg by Mitra Taj, Editing by Franklin Paul)