A record £82m was spent on locum doctors by NHS Scotland last year, according to new BBC figures.
NHS boards have spent £18m more than the previous year on recruiting locums because of a shortage to fill vacant medical posts.
The data obtained by BBC Scotland painted a grim view of boards spending a huge proportion of their budgets on cover staff; the Western Isles and Shetland spent up to a third.
In October Audit Scotland warned NHS boards were not able to meet their financial targets, with four boards requiring extra funding from the Scottish government to balance their books.
A spokesperson said the Scottish government had worked with NHS boards to provide medically trained staff, which meant more shifts were covered.
"It is the responsibility of NHS Boards to ensure they have the correct staffing levels in place to deliver safe and effective patient care.
"It is crucial that they do so in a sustainable way, ensuring that cover is in place for periods of high demand, or when staffing is reduced due to sickness", they said.
Since 2009, NHS Tayside increased its spending on agency locums by 1080%. The cost of using agency rather than internal cover staff is generally higher, reported the BBC.
NHS Tayside medical director Dr Andrew Russell said: "NHS Tayside, like all other health boards, has experienced difficulty in recruiting to certain specialities such as paediatric, psychiatry and anaesthetic doctors due to a national shortage of doctors in these areas.
"Our locum expenditure has, in the main, been due to filling these gaps."
He added: "The use of locum doctors is always a position of the last rather than the first option and the decision to employ a locum doctor is always made by a senior doctor in the organisation."