Professor David Nutt is one of the UK's leading neuroscientists.
Last year he suggested that LSD, ecstasy and cannabis were less harmful than alcohol and tobacco.
He was promptly sacked as the government's chief adviser on drug abuse. His removal from post raised questions about the use of scientific advice in government.
He talks to Stephen Sackur.
You can watch HARDtalk on Tuesday 25th May on the BBC News Channel at 0230, 0430 and 2330 BST and on BBC World News at 0330, 0830, 1530 and 1930 GMT.