Prescription drug denials by private insurers in the United States jumped 25% from 2016 to 2023, according to a new analysis of more than 4 billion claims, a practice that has contributed to rising public outrage about the nation’s private health insurance system.

The report, compiled for The New York Times by medical data company Komodo Health, shows that denial rates rose from 18.3% to 22.9%. The rejections went up across many major health plans, including the country’s largest private insurer, UnitedHealthcare.

The data offers a rare look into the largely hidden world of rejected insurance claims. While some government-funded health plans are required to publish their denial rates, most private insurers keep that information confidential. Komodo draws from private databases that collect denial details from pharmacies, insurers and intermediaries.

Experts who have studied denials said the skyrocketing costs of popular new weight-loss medications and greater automation of the claims process with artificial intelligence may have contributed to the rising rejection rates.

“The challenge for a lot of people accessing the health care system is that it is a morass of competing interests,” said Arif Nathoo, Komodo’s CEO. “At the heart of it is a patient experience that is really suboptimal.”

AHIP, the insurance industry’s lobbying group, laid some of the blame for denials on pharmacies, doctors and rising drug prices.

“Health plans approve the vast majority of claims they receive, and spending on prescription drugs continues to escalate,” said Chris Bond, a spokesperson for the group. “However, providers or pharmacies sometimes submit duplicative, inaccurate or incomplete claims that can result in an initial denial — a frustrating outcome for patients.”

While UnitedHealthcare has been singled out by the public, the Komodo data shows that five of the largest private insurers have similar rejection rates, all of which have increased since 2016. (The rate of one insurer, Humana, dropped in the last two years of the analysis.)

UnitedHealthcare declined to comment on the new data. Representatives for Aetna, Humana, Cigna and Anthem declined to comment on the data.