An experimental diabetes drug that may increase the risk of congestive heart failure and death should not be allowed on the market despite initial support from U.S. regulators, a consumer group said on Monday.

Public Citizen’s Health Research Group called on the U.S. Food and Drug Administration to hold off approving Pargluva, made by Bristol-Myers Squibb Co. and Merck & Co Inc., without “at the very least” a five-year clinical safety study.

The fate of the drug, know generically as muraglitazar, is already unclear.

Earlier this month, Bristol-Myers’ Chief Executive Officer Peter Dolan told Reuters it would decide in the next “several months” whether to continue working on Pargluva after the FDA said more data were needed.

In October the agency issued the companies an “approvable” letter asking for more heart safety information before the FDA made a final decision.

Company officials decided that could take five years to complete.

In a letter to the FDA sent last week, Public Citizen urged the agency to make sure the drug is safe before allowing its sale, especially since other diabetes drugs are already on the market.

“We question why a drug that an FDA medical officer has concluded causes dose-related edema, weight gain, and congestive heart failure needs to be approved for diabetic patients who are already at increased risk for cardiovascular adverse effects and who already have multiple drugs available to treat their conditions,” the advocacy group wrote.

Since the FDA’s initial decision, several studies have also called the pill’s heart risks into question, they said.

Representatives for Bristol-Myers could not be immediately reached for comment on the letter. Merck referred calls to Bristol-Myers.

Pargluva aims to lower blood sugar as well as some blood fats and, if approved, would be the first in a class of drugs called glitizars that also target fats.

An estimated 18 million Americans have diabetes. Obesity is a major cause of the most common form, type 2 diabetes, which prevents the body from properly using the hormone insulin to control blood sugar.