For 23 years, Diane Whitcraft injected herself every other day with Betaseron, a drug that helps prevent flare-ups from multiple sclerosis. The drug worked well, drastically reducing Whitcraft’s trips to the hospital. But as her 65th birthday approached last September, she made a scary decision: to halt the medication altogether.

With health insurance through her job, Whitcraft had paid a $50 or $100 monthly co-pay for the drug; she hadn’t even realized that the price of Betaseron had soared to more than $86,000 a year. Shopping around for drug coverage through Medicare, the out-of-pocket costs were mind-boggling: close to $7,000 annually.

“I was just feeling really bad that my disease was going to affect our retirement budget,” Whitcraft said. “You’re retired; you’re on a fixed income. And it just really was bothersome to me. I was doing this to us. This disease was doing this to us.”

Whitcraft’s dilemma highlights a growing problem with Medicare prescription drug coverage for seniors who take high-priced specialty drugs: There is no cap on how much they pay. Each prescription drug plan is structured a little differently, but people with very high drug costs almost inevitably enter what’s called the “catastrophic” phase of coverage. Then, they pay 5 percent of the list price of their drug – no small sum in an age of $10,000-a-month cancer drugs or, in Whitcraft’s case, a more than $7,000-a-month multiple sclerosis therapy.

The number of seniors who reach the catastrophic phase has almost doubled over a four-year period, to more than 1 million people in 2015, according to a new analysis by the Kaiser Family Foundation. That trend was driven in part by a new generation of high-priced hepatitis C drugs, but includes high out-of-pocket costs for people taking drugs for cancer, multiple sclerosis, schizophrenia and HIV.

The Affordable Care Act took steps to close the “doughnut hole,” the coverage gap where seniors have been on the hook for more of their prescription drug costs. But for a growing number, the doughnut hole barely matters. Their first or second prescription fill of the year might get them out of it, plunging them into a bigger problem – a phase of coverage where there’s no upper limit on how much they will pay.

“Once people blow through the doughnut hole and reach the catastrophic threshold, they continue to pay. And these costs are ticking up,” said Tricia Neuman, a senior vice president at the Kaiser Family Foundation. “While 5 percent coinsurance doesn’t sound like a lot, it can really add up for people who are taking extremely expensive medicines.”

The Kaiser study found that in 2015, the 1 million seniors who reached the catastrophic threshold paid an average of more than $3,000 out of pocket. One in 10 of them paid at least $5,200.

Neuman noted that the data, showing a huge increase in the number of people reaching the catastrophic threshold, wouldn’t even take into account people such as Whitcraft, who simply opt out and don’t fill prescriptions because of the cost. One possible policy solution would be to add a cap for out-of-pocket drug costs beyond a certain threshold – an idea that has been proposed in legislation.

Stacie Dusetzina, a cancer health services researcher at the University of North Carolina at Chapel Hill, said that the trend probably is driven by a combination of factors: more high-priced specialty drugs coming on to the market, price increases over time for existing drugs and more people taking expensive drugs.

The trend also challenges the pharmaceutical industry’s main argument in defending list prices – that those prices are misleading because they do not represent the secret rebates provided to insurers or reflect what patients pay. Seniors are paying coinsurance prices paid based on the list prices, not the secretly negotiated rebated price.

“This is why list prices matter, and rebates aren’t directly helping people needing specialty drugs,” Dusetzina said.

Whitcraft took her last dose of Betaseron on Jan. 5. So far, she hasn’t had another attack, but she knows the threat is always there. She said she wouldn’t have made the same decision to stop the drug if she were younger, and she wrestled with what to do.

This summer, she did something she thinks she should have done a long time ago. She wrote a letter to the chief executive of Bayer, the company that makes the drug.

Bayer spokeswoman Sasha Damouni said that the company goes through a series of steps before making a decision on how to price a drug, including discussions with doctors and patients. The company also assesses the product’s ability to reduce health-care costs by avoiding unnecessary hospitalizations.

But Whitcraft still doesn’t understand why her drug, which launched with a list price of about $11,500 more than two decades ago, costs so much today – a question she raised in her letter.

“It wasn’t filled with anger or anything; I just told him that I had quit the drug, and why. And I suggested someone must be very greedy,” she said. “It’s so wrong and so unfair – a drug that was marketed for the first time in 1993 . . . Why did the cost go up so much here?”

Whitcraft said she got a phone call from the company offering the drug at a discounted rate, months after she had come to the difficult decision to stop taking it. She wondered, if the company could offer her a discount on an individual basis, why they couldn’t just lower the price for everyone.