U.S. health care spending by the end of 2015 is expected to grow nearly 6 percent from last year, according to the Centers for Medicare and Medicaid Services (CMS). Expanded coverage under the Affordable Care Act (ACA) and the rising cost of prescription drugs are mostly to blame for the uptick, Health Affairs blog reports.

Many experts predicted that spending would rise this year because of health care reform. However, the huge jump in prescription drug prices in 2014—an increase of nearly 12.2 percent from the previous year—was largely unforeseen.

These experts have pointed to the introduction of new hepatitis C virus (HCV) treatments—such as Gilead Sciences’ Sovaldi (sofosbuvir), AbbVie’s Viekira Pak (ombitasvir/paritaprevir/ritonavir; dasabuvir) and Gilead’s Harvoni (ledipasvir/sofosbuvir)—as largely responsible for the increase.

About 150,000 people per year in the United States have used the HCV cures, which can cost upward of $90,000 per person for a standard 12-week course of therapy, costing the federal government nearly $14 billion in annual expenditures during the last two years.

However, while hep C spending remains very high—estimated to hit nearly $14.4 billion by the end of 2015—prescription drug costs overall seem to have leveled off over the last quarter of this year. Much of this spending slowdown could be attributed to many states’ prioritizing HCV treatment for only the sickest people, and limiting treatment based on a variety of exclusionary criteria.

Recent recommendations from medical advocacy groups like the American Association for the Study of Liver Diseases and the Infectious Diseases Society of America are now pushing for the treatment of nearly all diagnosed hep C cases. That alone makes health care spending predictions over the next few years highly unpredictable.