By Brian Wheeler
BBC News politics reporter
Admitting a youthful dalliance with soft drugs has almost become a rite-of-passage for Britain's politicians in recent years.
Opinions on drugs are sharply divided within Parliament
Everyone from Conservative chairman Francis Maude to home secretary Charles Clarke has owned up to puffing the occasional joint in their youth.
Provided the "experimentation" was brief, firmly in the past and subsequently regretted, the story is swiftly forgotten.
Yet - as would-be Tory leader David Cameron is finding out - the drugs issue still has the power to land politicians in hot water with the media and party colleagues.
Mr Cameron - unlike his three leadership rivals - has refused to say whether he has ever taken drugs or not.
Put on the spot by Observer journalist Andrew Rawnsley at a party conference fringe meeting, Mr Cameron said: "I had a normal university experience, if I can put it like that."
Pressed further on the subject in a later television interview, he said: "I did lots of things before I came into politics which I shouldn't have done. We all did."
Then, on Thursday, while admitting nothing specific on the issue, Mr Cameron told BBC One's Question Time: "I'm allowed to have had a private life before politics in which we make mistakes and we do things that we should not and we are all human and we err and stray".
Under normal circumstances, Mr Cameron might have been tempted to be more forthcoming about what he got up to in his student days at Oxford University.
But in the fevered atmosphere of a party leadership contest, his reticence is, perhaps, understandable. Mr Cameron cannot risk alienating potential voters.
He will also know, argues Mr Rawnsley, that far from ending speculation, owning up to recreational drug use - if that is indeed what he got up to in the past - would lead to more questions such as "what did he take?" and "how much?".
The danger, Mr Rawnsley adds, is that Mr Cameron will begin to appear "evasive" - an image at odds with the fresh, candid one he is trying is trying to project.
Mr Cameron has faced continuing questions over drugs
Several of Mr Cameron's Conservative colleagues - including leadership rival Ken Clarke - have backed his right to remain silent on the issue.
They argue that as long as his alleged indiscretions are in the past - and do not involve hard drugs or any other potential scandal - then they should remain his business.
But even with politicians on all sides increasingly calling for a "mature debate" on the issue - and previously taboo subjects such as homosexuality increasingly acceptable in Westminster - drugs remains a tricky area.
Drug abuse and the damage it causes to families, individuals and the economy is likely to remain near the top of the political agenda.
And the public remains sensitive to any whiff of hypocrisy - as opposed to something more exotic - emanating from its politicians.
When in 2000, then shadow home secretary Ann Widdecombe came up with the idea of £100 fines for people caught with even the smallest amount of cannabis, the policy was instantly undermined by a parade of shadow cabinet ministers owning up to past drug use.
Eight members came out as having used cannabis, including Francis Maude, shadow industry secretary David Willetts and shadow environment secretary Oliver Letwin.
Tim Yeo, then agriculture spokesman but now a backbencher, told the Times: "I was offered it on occasion and enjoyed it. I think it can have a much pleasanter experience than having too much to drink."
His career did not seem to suffer a permanent hangover, as he later became party spokesman on both health and education.
Labour's Charles Clarke admitted in 1997, the year he became an MP, that he had taken drugs "a couple of times in my late teens". He is now home secretary.
In 2002, having left politics, the late Mo Mowlam wrote in her autobiography that she had tried cannabis.
Then drugs czar Keith Halliwell said he advised Ms Mowlam to come clean about her youthful experimentation after learning she was involved with drugs at university.
Altogether, at least 31 current MPs have gone public over previous drugs use.
But like much else in politics, a lot depends on the timing and context of any admission - and whether there has been attempt at a cover-up.
The highest-profile politicians - the leaders or would-be leaders - still tend to remain coy, or adamant in denial.
Former US President Bill Clinton famously admitted using cannabis, but not inhaling.
Prime Minister Tony Blair was criticised in some quarters for inviting unashamed drug user Noel Gallagher to Number 10.
But on the subject of whether he has ever taken illegal drugs himself, Mr Blair has remained silent.
Whether Mr Cameron is able to follow his example will be tested over the next few days and weeks.