David Cameron has defended his right to a private life "before politics" and refused to say whether or not he had ever tried hard drugs.
Asked on BBC One's Question Time whether he had taken Class A drugs, the Tory leadership contender said: "I have said all I want to say about this."
He said everybody was allowed to "err and stray" in their past.
And he told the audience he would not bow to a "media-driven agenda" to "dig into politicians' private lives".
'Typical student experience'
Leadership rival Ken Clarke - who told a meeting on Wednesday he had never taken cocaine - leapt to Mr Cameron's defence saying he was a "decent guy" and questions about drugs were just "something to sell newspapers".
Thursday: Nominations closed. Hustings for the Contact group of Tory spouses
Monday: Possible hustings for all Tory MPs
Tuesday: First round of voting by MPs
Thursday: Final round of voting by MPs
Early November: Tory members start voting on final two candidates
6 December: Result expected
But Mr Clarke also attacked his younger rival's relative lack of political experience, saying the shadow education secretary has "never actually played any role in national politics".
"One speech and a fortnight's publicity and he is up there, with people saying he is now a prime ministerial candidate," Mr Clarke told BBC Radio Nottingham.
Mr Cameron, who is favourite at the bookmakers to snatch the Tory crown, was initially asked at a fringe meeting at this month's Conservative party conference if he had ever taken drugs.
The 39-year-old frontbencher told the meeting he had had a "typical student experience", adding later in a television interview: "I did lots of things before I came into politics which I shouldn't have done. We all did."
But the question has refused to go away after Mr Clarke and then Liam Fox both confirmed they had never taken Class A drugs.
Tackled about the issue on Thursday's Question Time, Mr Cameron said: "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."
He said it would be sad to have politicians who were "just machines".
David Cameron 4/6
David Davis 9/4
Liam Fox 9/1
Ken Clarke 10/1
"I didn't spend the early years of my life thinking: 'I better not do anything because one day I might be a politician' because I didn't know I was going to be a politician.
"And I haven't answered the question about drugs because I think that's all in the past and I don't think you have to answer it," he added.
Meanwhile, Mr Cameron was revealed in the London Evening Standard to be helping a member of his family who is receiving treatment for heroin addiction.
On Monday, Mr Cameron told sixth formers at a London school he had seen people close to him "wreck their lives" with drugs and he wanted to reduce drug abuse in the UK.
Tory MPs vote next Tuesday, when the candidate with the least votes will drop out. The top two in a second vote, on Thursday, then go to a run-off vote of party members.
Mr Cameron remains favourite with the bookmakers to win the contest but David Davis appears to have a big lead among MPs, with 67 publicly backing him, compared to 34 for Mr Cameron.
Dr Fox is believed to have 16 confirmed backers among MPs, with a similar number backing Mr Clarke.