A CVS Health Corp health plan that uses an outside drug pricing group to help it decide whether to cover certain new medicines has gained little traction with customers, according to its top medical executive, and has drawn fierce criticism from patient advocacy groups.

The company has held back on marketing the pharmacy benefit plan while it talks to these groups, CVS said. The plan, launched a year ago, is based on analyses by the Institute for Clinical and Economic Review (ICER), a Boston-based group that assesses effectiveness of drugs to determine appropriate prices.

Using ICER’s cost effectiveness assessment, CVS decides whether to include second or third medicines entering the market if there are already similar ones in the plan.

Opposition to the CVS plan is part of much broader concerns cited by drug companies and advocacy groups, many of which receive funding from the pharmaceutical industry. Some say that ICER’s analysis based on additional years of “quality life” gained from a given treatment is arbitrary and disregards the costs of drug development and patient needs.

More than 50 groups, including drugmakers, PhRMA, the industry’s main lobby group, and other advocacy groups, have provided comment during a public input period included in a review by ICER of its assessment methods. Many asked ICER to eliminate price recommendations from its efficacy analyses.

ICER has defended its methods, which are based on a widely-used cost effectiveness analysis.

The soft rollout of CVS’ ICER-related product comes as employer health plan sponsors – its biggest clients – are showing increased concern over their cost for new high-priced drugs, and are considering refusing to pay for them at all, CVS Chief Medical Officer Troy Brennan said in a recent interview.

If corporate customers follow through on that threat, CVS said it could change tactics with the plan.

Earlier this year, Novartis AG launched Zolgensma, a more than $2 million gene therapy for a rare but deadly disease called spinal muscular atrophy.

The new CVS program, cited as an example of ICER’s growing influence on U.S. drug pricing, would not apply to such a breakthrough treatment. It is a tiny plan by CVS standards as the company manages pharmaceutical benefits for more than 102 million people and also owns Aetna insurance and a national pharmacy chain.

The plan’s scope is limited to so-called me too drugs, those where similar effective treatments already exist, and aims to pressure drugmakers to set lower prices. For example, two of three very similar drugs for migraine approved in 2018 could have been excluded, but drugmakers set prices ICER deemed cost effective.

Large healthcare consultant and brokerage Mercer said it has begun to field similar concerns.