New enrollees on the Obamacare exchanges are spending less overall on medications this year than in 2014, but their spending on high-cost specialty drugs rose.

The analysis from the pharmacy benefits manager Express Scripts offers insights into the health of Obamacare enrollees, a key indicator of the sustainability of the healthcare law. If too many enrollees are sick, it could force insurers to raise costs or drop out of the health exchanges.

The company, which handles drug benefits for insurers, found that in the first quarter the number of new exchange plan enrollees who used at least one prescription drug fell by 18 percent.

However, spending on high-cost specialty drugs to treat life-threatening conditions this quarter rose by 24 percent compared with 2014. Express Scripts lays the blame on hepatitis C drugs that can cost consumers nearly $100,000 for a treatment regimen.

Spending on just hepatitis C drugs rose nearly 100 percent compared to the first quarter of 2014. A reason for the huge spike was in early 2014 high-cost treatments Sovaldi and Harvoni from Gilead Sciences had not hit the market.

Express Scripts previously battled unsuccessfully with Gilead Sciences, manufacturer of the high-cost Sovaldi and Harvoni, on lowering the price of the breakthrough hepatitis C cures. Eventually the pharmacy benefits manager declined to cover the treatments.

The data offers a look into the health of who is enrolling in the exchanges.

"While it is encouraging to see data that suggests healthier Americans are enrolling in exchange plans, the research reminds us that many of these patients use this benefit to manage serious, chronic illnesses," said Julie Huppert, vice president for healthcare reform at Express Scripts.