Can't afford new dentures or a doctor's visit? Your physician may give you a credit card application right there in the office.

But these cards often charge 25 to 30 percent interest once a promotional interest-free period expires.

The cards are becoming more common, according to Roger Bertling, a professor of law at Harvard University.

"If you don't pay it back during that period of time, often the interest goes back to the beginning, goes back retroactively, which a lot of people don't understand," he said. "I think they can be beneficial, but I think people really have to know what they are getting into -- and I wonder if these aren't the kinds of things that regulators need to take a little closer look at."

It's hard for patients to say "no" if otherwise they can't afford their medical bills, said
Edmund Mierzwinski, federal consumer program director for the U.S. Public Interest Research Group.

"It's absolutely the case that when you've got a doctor selling a credit card, you're mixing apples and oranges. They may not explain the terms properly and the terms may be buried.

"The company that is providing the card to the doctor may include all the terms required by law, but you have to understand another thing," he said. "Consumers who are seeking medical treatment want medical treatment."

Medical credit cards first appeared about a decade ago. The American Medical Association has not taken a stance for or against them.