(Image: EPA/Camera Press/Bob Pearson)

No wonder they look pleased. Darren Austin and his son Tyler from Decatur, Georgia, are queuing to buy the first legally available recreational marijuana in the US, from the Denver Discreet Dispensary in Colorado.

From 1 January, residents of Colorado aged 21 or older became the beneficiaries of the first law anywhere in the world to legalise marijuana for recreational use. Like 17 other US states over the past decade, Colorado has allowed the sale of marijuana for medical purposes, but the extension to recreational use is unprecedented.

So far, 136 stores have obtained licences to grow and sell marijuana. Residents of Colorado are allowed to buy up to an ounce (28 grams) in one go – enough to make about 60 joints. People visiting from outside Colorado, like Darren and Tyler, are allowed up to a quarter of that amount.

To keep tabs on what is produced, every plant will be radio-tagged throughout its life cycle. The hope is that the legally available produce will ultimately undercut and displace illegal supplies, reducing the crime that goes with it.