THE weeds are spreading. Although the federal government still considers the use of cannabis a criminal offence, nearly half the states of America have legalised it in some form.

Most states sell it only for medical purposes, often broadly defined. But four states—Alaska, Colorado, Oregon and Washington—and the country's capital have gone further, legalising the drug’s recreational use. Legal weed is more expensive than the black market variety, but it is better value: three times more potent, on average, and only about 50% dearer. A slim majority of Americans now favours legalisation, but support varies widely. Whereas in the west the legalisers outnumber the prohibitionists easily, in the north-east opinion is split almost down the middle.