From the article...

According to existing British and U.S. drug policy, alcohol and tobacco are legal, while cannabis and Ecstasy are both illegal. Previous reports, including a study from a parliamentary committee last year, have questioned the scientific rationale for Britain's drug classification system.

"The current drug system is ill thought-out and arbitrary," said Nutt, referring to the United Kingdom's practice of assigning drugs to three distinct divisions, ostensibly based on the drugs' potential for harm. "The exclusion of alcohol and tobacco from the Misuse of Drugs Act is, from a scientific perspective, arbitrary," write Nutt and his colleagues in The Lancet.