David Cameron has defended the coalition's radical NHS reforms, insisting there was no "quiet life option" to ensure patients receive the best treatment.
The Prime Minister said the health service's performance had fallen behind other systems in Europe, even though spending was at similar levels.
Some experts have criticised the speed at which the changes - which will see GPs take control of commissioning care - are being pushed through.
But Mr Cameron told the BBC Radio 4 Today programme: "Firstly, it's right to start the process of change now. I've looked back on the previous government - they waited too long before introducing changes that were necessary and that would improve services.
"The second point is this is being introduced steadily. We are not asking GPs to take on new responsibilities for two years."
The premier said there was "enthusiasm" among the medical profession for the shake-up.
He also denied he had been bounced into supporting the measures by Health Secretary Andrew Lansley despite the Tories pledging to give the NHS more stability when in opposition.
"In an NHS where, at the moment, drug bills are going up, the population is ageing, extra costs are being introduced, there is not a quiet life option," Mr Cameron added.
Mr Cameron went on: "I think if we just carried on as we are... I think we would face a really big crunch in two or three years' time.
"It is necessary because we have fallen behind the rest of Europe. We have spent similar amounts of money, we are more likely to die of cancer or heart disease. I do not think we should put up with being second-best. We should aim to be the best."
