More than a half-million U.S. patients had medication costs in excess of $50,000 in 2014, an increase of 63 percent from the prior year, as doctors prescribed more expensive specialty drugs for diseases such as cancer and hepatitis C, according to an Express Scripts report released on Wednesday.

Of the estimated 575,000 Americans who used at least $50,000 in prescription medicines last year, about 139,000 used at least $100,000 worth of medication, nearly triple the 47,000 who hit that mark in 2013, the report said.

The total cost to health plans for U.S. patients with prescription drug expenses in excess of $50,000 was $52 billion in 2014, Express Scripts said in its report: "Super Spending: Trends in High-Cost Medication Use."

"These patients are overwhelmingly taking specialty medications, and have multiple (health problems), prescriptions and prescribers," Glen Stettin, Express Scripts Holding Co's head of clinical, research and new solutions, said in a statement.

Health insurance covered 97.4 percent of the drug expenses among patients prescribed at least $50,000 worth of medicines in 2014, according to Express Scripts, the largest U.S. pharmacy benefit manager. By contrast, patients with less than $1,000 in total prescription medication costs paid 35 percent of the bill out of pocket.

Among baby boomers aged 51 to 70 in the high-cost category, 77 percent were being treated for hepatitis C, for which costly new cures were introduced last year. Fifty percent were being treated for cancer, for which several expensive new drugs were approved in the last two years. The new drugs for those diseases alone can cost upwards of $90,000 per patient.

About 60 percent of patients in the super-spending report were taking at least 10 medicines from at least four different prescribers.

"Patients in these highest-spend categories are treating a complex condition along with other more common chronic conditions, such as diabetes or depression," Stettin said.

Anti-depressant use among those also taking a specialty medicine was more than twice the national average, the report found.

The report looked at prescription drug claims of 31.5 million Americans with either commercial health insurance or coverage through federal and state Medicare and Medicaid plans.