If someone were to invent a perfectly safe ecstasy pill, what would be done about it? It's the sort of scenario clubbers like to speculate about, usually at around 6am, a little the worse for wear after a big night out. It's less common to hear it from a neuropsychopharmacologist and former government scientist – but it is, Professor David Nutt says earnestly, "the key question". So what does he think the government would do?