TRENTON, N.J. — U.S. spending on prescription drugs rose 8.5 percent last year, slightly less than in 2014, driven mainly by the growing use of ultra-expensive new drugs and price hikes on other medicines.

A report from data firm IMS Health estimates patients, insurers, government programs and other payers spent a combined $309.5 billion last year on prescription medicines.

The IMS Institute for Healthcare Informatics is forecasting that annual increases in U.S. prescription drug spending will slow to 4 to 7 percent through 2020, after rising around 10 percent in each of the past three years. It predicts spending will reach $370 billion to $400 billion in 2020.

The totals are based on net prices paid after deducting discounts and rebates that manufacturers give to insurers and other payers.

In prior years, IMS based its report list prices before those deductions.

The report comes amid growing criticism of unaffordable drug prices from patients, doctors, insurers, Congress and presidential candidates, who have pledged to rein in prices. Insurers have been trying to limit prices, demanding bigger discounts to cover many drugs, but have less clout on drugs with little or no competition.

Drug spending keeps growing due to factors including rising prices, fewer blockbuster drugs getting new generic competition and a 10 percent jump last year in the number of prescriptions filled, to nearly 4.4 billion. In prior years, total prescriptions dispensed edged up about 2 percent.

“We have more people with health insurance (now), and that could be a reason we’re having more uptake in brand-name drug use,” said Murray Aitken, the institute’s executive director.