Many of these cards initially charge no interest for a promotional period, typically six to 18 months, an attractive feature for people worried about whether they can afford care. But if the debt is not paid in full when that time is up, costly rates — usually 25 to 30 percent — kick in, the review by The Times found. If payments are late, patients face additional fees and, in most cases, their rates increase automatically. The higher rates are often retroactive, meaning that they are applied to patients’ original balances, rather than to the amount they still owe.

For patients, the financial consequences can be dire.

Ms. Gannon said she was happy with her dental care, despite the cost, and there was no suggestion that Dr. Knellinger had done anything wrong. But attorneys general in a several states have filed lawsuits claiming that other dentists and professionals have misled patients about the financial terms of the cards, employed high-pressure sales tactics, overcharged for treatments and billed for unauthorized work.

The New York attorney general’s office found that health care providers had pressured patients into getting credit cards from one company, CareCredit, a unit of General Electric, which gave some providers discounts based on the volume of transactions. Patients, the investigation found, were misled about the terms of the credit cards, and in some instances, duped into believing that they were agreeing to a payment plan with dental offices when, in fact, they were being pushed into high-cost credit.

In June, CareCredit reached a pact with Eric T. Schneiderman, the New York attorney general, to improve protections for consumers, and a spokeswoman said the company “does not incentivize providers to have patients open accounts” or give referral fees to providers.

In Ohio, the attorney general has sued the operators of several hearing aid clinics, claiming that they misled customers about using medical credit cards to pay for batteries and warranties.

Cameron P. Kmet, a chiropractor in Anchorage, Alaska, said he had stopped offering medical cards. “One missed payment can really ruin a patient’s life,” he said. Mr. Kmet now runs a company that administers payment plans directly between providers and patients, with annual interest rates around 8 percent.

Regarding medical credit cards, Mr. Kmet said he had urged providers to ask themselves “whether this is something that you would recommend to a family member or friend.” The answer, he said, is usually no.