Colorado's marijuana experiment has been an empirically rousing success thus far, with crime down and tourism up, and now the state has collected so much money in tax from sales of pot that it might be legally obliged to give some back.

The state constitution puts a cap on the amount of tax money that can be taken in before some has to be returned, meaning Coloradans could see a share of the $50 million generated by sales of recreational cannabis.

It's such an uncommon situation that both Democrats and Republicans are in agreement on it - both insist that there is no point in returning the money to taxpayers, not something you usually hear the GOP saying.

420 festival: Cannabis Cup celebrations in Denver, Colorado Show all 7 1 /7 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver Opponents of the legalisation of cannabis in Colorado have seized on what is thought to be two recent cannabis-related deaths AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver The organisers of Denver's 420 Rally estimated that about 37,000 people went to the free festival this year AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver With the Colorado state capitol building visible in the background, partygoers dance to live music and smoke pot on the first of two days at the annual 4/20 marijuana festival in Denver AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver The annual event is the first 420 marijuana celebration since retail marijuana stores began selling in January 2014 AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver The term 420 was invited by a group of high school friends who used it as a code for smoking pot AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver This year was the first time in 11 years 420 fell on the same day as Easter Sunday AP 420 festival: Cannabis Cup celebrations in Denver, Colorado 420 festival in Denver The Denver 420 Rally was expected to be the biggest of its kind Rex Features

"I think it's appropriate that we keep the money for marijuana that the voters said that we should," said Republican Senate President Bill Cadman.

"This is a little bit of a different animal. There's a struggle on this one," added Sen. Kevin Grantham, one of the Republican budget writers.

Coloradans may be asked to vote on making marijuana exempt from the tax cap - the money instead being ploughed into more drug education and police training to spot stoned drivers, as marijuana legalisation is designed to pay for itself without dipping into general taxes.