US Edition In an interview for Telegraph TV, the candidate for London mayor responded to a question from a reader about his personal views on the use of marijuana by sufferers of chronic conditions such as arthritis.
Mr Johnson said: "I have thought about this for a little bit, but I haven't looked at all the evidence and talked to the police about it in a way I would before giving more than an extempore answer.
"However, I do think there is a case when cannabis is being used to alleviate severe and chronic pain that the law should be flexible."
The comments will irritate his senior Tory colleagues. Two weeks ago, David Davis, the shadow home secretary, described government policy as being in "chaos" amid reports that Gordon Brown was considering reclassifying marijuana as a Class B drug.
Yesterday, Mr Johnson also vowed to clean up London politics as he answered questions from readers of telegraph.co.uk a week before polling day. He fielded queries about law and order, the Olympics and transport.
However, it was the topic of Ken Livingstone, and Mr Johnson's claim that the Labour Mayor has mismanaged the capital and squandered taxpayers' money while running a "corrupt" administration, that dominated the exchanges.
Asked how he would reduce the size of local government in London and cut red tape, Mr Johnson said: "I am going to make some sensible economies at City Hall by axing some of the current mayor's loony tunes expenditure – like the European fund for Marxists and anarchists. He has absolutely no concept of taxpayer value, which is why on day one, I am going to do the following – put every pound that I spend on behalf of the people of London on the web, so that the public can consult it, and we end the disgraceful cronyism and corruption of the current system."
Asked about the spiralling cost of the Olympics, Mr Johnson again criticised the mayor and his campaign chief, the Olympics minister Tessa Jowell, saying: "I will not allow Londoners to pay a penny more for the Olympics, and I will ensure from day one that we have proper financial controls and accountability.
"Tessa Jowell and Ken Livingstone are simply not adequate protectors of the public purse." The Conservative candidate accused Mr Livingstone of having a secret plan to put up fares on the buses, trains and the Tube, while promising that he would transform London's transport system.
He said: "Above all, I will not deceive the London public about fares. It is an utter scandal that this mayor is going into the election on a promise not to raise bus and Tube fares – when it has just emerged that he has secretly agreed they will have to go up.
"He is treating the voters and the travelling public with contempt." Meanwhile, Mr Livingstone appeared to entertain for the first time the prospect that he will lose the election – and even offered his rival advice on becoming the new mayor.
He said: "If I don't win, come May 6 I will be taking the kids to school and starting a book on my last eight years as mayor. Boris, because he has never been a council committee chairman or chair of a local authority, or involved in any other local government, would still have a lot to learn even after the election. My advice would be don't rush to make rapid change."