It depends what you consider your "credit rating" to be. If you earn £10,000 pa and have a £3,000 credit card and a £2,000 overdraft, then being in that overdraft will affect your ability to get further credit, as lenders may be reluctant to increase your income:credit ratio below 2:1 if you are using, say, 90% of your currently available credit. However, utilising the overdraft will build a history of you managing credit and as such will work to improve your rating long-term.