West Sussex health bosses could find themselves £5 million in the red at the end of this financial year.
The newly formed West Sussex Health and Social Care NHS Trust is working on an action plan to bring its finances under control.
Finance experts say it is unlikely the trust will manage to break even by the end of March but are hopeful of balancing the books in two years time.
The trust, which was created on April 1, is responsible for all mental health and most learning disability services for the whole of West Sussex.
Finance director Spencer Prosser said the trust had inherited two main problems from previous organisations.
These were the high number of patients using beds in the private sector and the large number of medical vacancies.
He said: "The shortage of staff means we have to bring in expensive agency workers, who add significantly to our costs.
"In the light of this we will be concentrating on a recruitment campaign to bring in more staff."
Peter Atkinson, spokesman for public sector union Unison, said: "We knew the new trust was going to inherit some financial problems as the old West Sussex Health Authority was always struggling.
"We feel very strongly that the slate should have been wiped clean by the Government."
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