One hundred years ago, the US convened the International Opium Conference. This meeting of 13 nations in Shanghai was the beginning of global drug prohibition.

Prohibition slowly became one of the most universally applied policies in the world. But a century on, international support for this blanket drug policy is slowly but inexorably unravelling.

In January, Barack Obama became the third US president in a row to admit to consumption of cannabis. Bill Clinton had admitted using cannabis but denied ever inhaling it. George Bush was taped saying in private he would never admit in public to having used cannabis. When Obama was asked whether he had inhaled cannabis, he said: ''Of course. That was the whole point.''

Obama has candidly discussed his drug use. ''Pot had helped, and booze; maybe a little blow [cocaine] when you could afford it.'' He has also admitted the ''war on drugs is an utter failure'' and called for more focus on a public health approach.

In February, a Latin American drug policy commission similarly concluded that the ''drug war is a failure''. It recommended breaking the ''taboo on open debate including about cannabis decriminalisation''. The same month, an American diplomat said the US supported needle-exchange programs to help reduce the transmission of HIV and other blood-borne diseases, and supported using medication to treat those addicted to opiates.