Professor David Haslam, former RCGP chairman and president, defended NICE's decision-making on new drugs, saying it was 'an inescapable and global truth that funding for healthcare is limited'.
Professor Haslam also questioned why drug development costs are so high.
NICE has faced years of criticism over decisions not to approve new treatments based on high cost.
The government set up the Cancer Drug Fund in 2011 to give patients access to treatments blocked or yet to be evaluated by NICE. The scheme has been extended to 2016.
Speaking at the NICE annual conference in Birmingham on Tuesday, Professor Haslam said patients in other countries may have access to 'hugely expensive' treatments blocked by NICE.
But cancer patients in the US are two-and-a-half times more likely to declare bankruptcy than the rest of the population, he argued. 'Isn't bankruptcy as much a side-effect of that treatment as nausea?'
He went on: 'You can only ignore real costs if you're not dealing with real money. Real money is spent by someone. Once spent, it's not available to be spent on something else.
'If it's patients who end up bankrupt, or healthcare systems that cannot afford to offer care equitably, because of huge expense in one particular area of care, that ignoring fiscal value as well as clinical value carries huge risk, to patients as well as systems.
'If we keep paying unsustainable prices, what happens? Something's bound to crash.'
Professor Haslam questioned the cost of new drug development. 'Why is it so expensive? It seems to me currently no one really knows, outside the board of a pharma firm, how it evaluates a drug's cost.
'I think if we can understand more about this, it would be of real benefit.'