A survey by Unison of more than 3,300 NHS employees also showed that just over half are overdrawn every month.
The study, published ahead of a four-hour strike by health employees on Monday, found that extra jobs included lifeguard, tourist guide, hairdresser, driving instructor, and dog groomer.
Some had started their own business, or did extra hospital shifts, complaining that they could not live on their NHS salary.
Nurses, midwives, ambulance drivers, hospital porters and other health workers will walk out from 7am on Monday in protest at the Government’s controversial decision rejecting a recommended one per cent pay rise for all NHS staff.
Health chiefs in parts of Yorkshire are warning of disruption to hospital and ambulance services.
Unison said its poll showed that almost two out of five health workers relied on credit cards, while 13 per cent had resorted to payday loans.
Head of health Christina McAnea said: “The Government is refusing to acknowledge that there is a real poverty problem affecting NHS workers.
“NHS workers work day in day out to provide vital care and support for millions of patients so they deserve fair pay. A full-time hospital cleaner should not have to deliver pizzas after work to make ends meet.”
The Department of Health said a pay rise risked frontline jobs and it was “disappointed” industrial action was taking place.