Some Scottish NHS boards may not be able to balance their books this year, Audit Scotland has warned.
The public spending watchdog said the health service needed to make unprecedented savings in 2016/17.
And it said NHS funding was not keeping pace with increasing demand on the service.
It also said NHS Scotland met only one of its eight key waiting time targets last year - the drug and alcohol treatment standard.
Audit Scotland's annual report on the NHS said performance had actually declined in six of the eight targets over the past four years - although the target for cancer patients waiting no more than 31 days for treatment was missed by just 0.1%.
However, Audit Scotland stressed that the health service had managed to make some improvements despite the "extremely challenging financial position".
Health Secretary Shona Robison insisted the Scottish government had made "significant improvements" and has a strategy to change the way services are delivered.
But opposition parties said the report painted a picture of an NHS that was "failing", and called for urgent action towards creating a "sustainable NHS".
Overall, the country's 14 health boards reported total savings of £291m last year - which had left some needing to use short-term measures to break even.
But the figure is expected to rocket to £492m in the current financial year, Audit Scotland said.
It said: "NHS boards are facing increasing costs each year, for example drug costs increased by 10%, allowing for inflation, between 2012/13 and 2014/15.
"NHS boards will need to make unprecedented levels of savings in 2016/17 and there is a risk that some will not be able to achieve financial balance."
The total health budget in 2015/16 was £12.2bn - an increase of 2.7% in real terms from the previous year, and representing 40% of the Scottish government's entire budget.
But the report said this increase was not enough to keep pace with increasing demand, rising costs and the needs of a growing and ageing population.
This strengthens the case for changing the way services are delivered, the report added.
It said there had been improvements in the way health services are delivered over the past decade, and reductions in the time that patients need to wait for hospital inpatient treatment.
There have also been improvements in overall health, life expectancy, patient safety and survival rates for a number of conditions, such as heart disease.
The report said: "However, boards are struggling to meet the majority of key national standards and the balance of care, in terms of spending, is still not changing.
"It is difficult balancing the demand for hospital care, alongside providing more care in the community. Boards need to ensure they maintain high-quality hospitals, while investing in more community-based facilities."
It also highlighted the ageing NHS workforce, and difficulties in recruiting and retaining staff in some areas, as well as the increased spending by health boards on temporary staff.
Caroline Gardner, the Auditor General for Scotland, said: "The Scottish government has had a policy to shift the balance of care for over a decade but despite multiple strategies for reform, NHS funding has not changed course.
"Before that shift can occur, there needs to be a clear and detailed plan for change, setting out what the future of the NHS looks like, what it will cost to deliver and the workforce numbers and skills needed to make it a reality."
Reaction to the report
Health Secretary Shona Robison: "Under this government there have been significant improvements in the performance of the NHS, the safety of patients, overall life expectancy and survival rates for conditions such as heart disease, supported by a real terms increase in spending on our NHS."
She said staffing levels and the health budget were at their highest levels, and highlighted the government's clinical strategy to shift the balance of care and its workforce plan to ensure adequate staffing for the future.
Ms Robison added: "These are changes that will help to equip our health service to continue to deliver excellent and safe care for the people of Scotland in the years ahead."
Royal College of Nursing Scotland director Theresa Fyffe: "RCN Scotland has been warning of the increasing pressures on NHS boards and integration authorities for some years now and it is clear from the Auditor General's report that we need to change the way we work in order to meet the increasing demand on services.
"How many more reports will be published by Audit Scotland before action is taken? Patients, staff and families deserve a decisive response from the health secretary."
Scottish Conservative health spokesman Donald Cameron: "This is a damning report, and shows the dire state that our NHS is currently in.
"The SNP need to take responsibility for this and start creating a sustainable NHS that provides the right support and is properly staffed."
Scottish Labour health spokesman Anas Sarwar: "The scale of mismanagement in our health service revealed by this report is utterly staggering. On every key test the SNP is failing.
"SNP Health Secretary Shona Robison must make an emergency statement to parliament."
Scottish Greens health spokeswoman Alison Johnstone: "We know that the Scottish government agrees with the Greens' policy on greater preventative spending, but ministers need to turn warm words into real action.
"By preventing ill-health, we can ease the pressure on our NHS and if we invest in good health now, through everything from warm homes and safe walking and cycling routes to good food and a living wage, we can lighten the burden on health services."
Liberal Democrat MSP Alex Cole-Hamilton: "The Audit Scotland report makes it clear that while SNP ministers talk a good game, they have utterly failed to shift the balance of care and put the NHS on a sustainable footing for the future."