To Cathy L. Morgan, a divorced mother trying to pay for braces for her 12-year-old daughter, the idea of a specialized medical credit card sounded ideal. Morgan expected to pay for the $5,300 treatment in monthly installments of $138 spread over four years, at an interest rate of 11.9 percent.

The Clearwater, Fla., real estate agent said that Debbie Fox, the wife and treatment coordinator of orthodontist Donald M. Fox, urged her to apply for the CareCredit card offered by GE Money Bank. Fox’s practice didn’t offer an extended payment plan, Morgan said, and the card seemed the only way to pay for braces, given her $30,000 annual income, which had plummeted in the depressed housing market.

Once largely the province of cosmetic dentists and plastic surgeons, the credit cards (also offered by Capital One, Citibank and Chase) are being used to finance necessary medical and dental treatment. The specialized cards represent an effort to capture part of the $45 billion in out-of-pocket medical expenses charged to credit cards, a figure that consultants at McKinsey and Co. predict may triple by 2015.

Providers benefit by getting paid more quickly. Consumers, depending on the card’s terms, receive an installment loan that may come with no annual fee or no interest if paid back within a promotional period.

GE Money spokeswoman Cristy F. Williams said that most of the 7 million Americans who have used CareCredit found it to be “a good money management tool. People value CareCredit in these times.”

But some consumer advocates say that medical credit cards can ensnare unwary or desperate consumers who do not understand the terms or realize that one or two missed or late payments can push the interest rate to nearly 29 percent. And consumers who charge medical expenses lose the ability to negotiate with the provider, noted Boston health advocate Mark Rukavina.

Claudia Lennhoff, executive director of Champaign County Health Care Consumers, an Illinois advocacy group, regards the cards as “essentially predatory lending” because patients may feel pressured to apply by health-care providers or feel they have few other options to finance a root canal or other urgent treatment.

Morgan said she encountered problems soon after CareCredit approved her for a card in December 2006. She said she made one payment late because the bill didn’t arrive. She soon discovered that her interest rate had jumped to 28.99 percent.

Morgan said she called the company to protest but “got nowhere” and ended up paying additional fees and interest charges. She has switched orthodontists and paid off the card in December 2007 with a loan from a friend.

“I feel very misled,” she said, adding that Fox and his wife knew she was a single mother with a low income.

Neither of the Foxes could be reached for comment. His office is closed, and he did not respond to several e-mails and telephone messages.

Williams said GE Money has no record of calls or letters from Morgan until shortly before she paid off her account, and no history of complaints involving Fox. She said that Morgan had an “inconsistent” history involving more than one missed payment, which resulted in the higher interest rate.

“We’re very flexible and strive to be compassionate and fair,” Williams said.