It sounds harmless.

Just sign your name, and a friend or relative will get a much-needed credit card. Like most things in life, however, being a co-signer is not that simple.

Financially, "co-signing is probably the worst thing you can do," says John Ulzheimer, president of consumer education for SmartCredit.com.

Judging by the complaints and lawsuits made by co-signers, it also seems to be one of the least understood arrangements, he says. When friends and relatives co-sign, they often don't realize the new card debt is also theirs -- 100 percent.

Often, the potential co-signer has a relationship with the wannabe account holder. That, too, can be jeopardized, says Ulzheimer.

With the potential for complications, personal and financial, "there are too many downsides," he says.

Even good character is no guarantee, says Todd Mark, vice president of education for Consumer Credit Counseling Service of Greater Dallas. Especially lately, his group has seen plenty of people with fine character who have had to default, he says. That can leave co-signers holding the bag.

"It's not worth risking your own financial security," he says.

Is someone asking you to co-sign for a credit card? Here are five facts you need to know before you decide.