His successor, George Bush, while acknowledging a near-alcoholic past, refused to say whether he had ever taken illegal drugs. In 1999, he said: "I've told the people of this country that, over 20 years ago, I made some mistakes when I was younger. I've learnt from those mistakes."

But a generation later there seems to be less bashfulness about the topic and controlling the message appears to be the key.

Barack Obama, the Democratic senator who has formally announced his bid to become America's first black president, long ago admitted taking both cannabis and cocaine while he was a teenager. In a 1995 memoir, Dreams From My Father, he wrote: "Pot had helped, and booze; maybe a little blow when you could afford it. Not smack [heroin] though."

Mr Obama has also stressed his activities were part of a youthful behaviour he has long left behind and which he does not condone. In an interview with The State Journal-Register newspaper of Springfield, Illinois, he said: "I was a confused kid and was making a bunch of negative choices based on stereotypes of what I thought a tough young man should be. Those choices were misguided, a serious mistake. Growing up to be a man involves taking responsibility. By the time I was 20, I was no longer engaged in any of this stuff."