Are you planning to buy a home, refinance a home or take out a loan for a large purchase such as a home, car or medical care? Lenders will look at your debt load. The co-signed account included.

"That account will impact your score no differently than if you were the only person on that account," says Barry Paperno, consumer affairs manager with myFICO.com. As far as creditors and potential creditors are concerned, "it is your account," he says.

Every consumer can only handle so much debt. If this card pushes you into the danger zone in the eyes of your own creditors, you risk paying higher rates on your own credit cards, higher rates on future loans and even being denied credit or having your credit lines cut.

A co-signed card also has the potential to drop your credit score, says Ulzheimer.

That's because credit-scoring formulas look at how much available credit you have and how much you use each month. The less you use, the better, he says.

Even if your account holder pays the bill on time every month, if that person is using a chunk of the available credit, it could lower your score, says Ulzheimer.