A U.S. expert panel assessing Novo Nordisk’s ultra long-acting insulin, degludec, will study its heart safety, raising a potential obstacle to approval in the world’s top market.

The panel will meet November 8 to focus on the issue, “as meta-analyses of several clinical trials suggest an excess risk for cardiovascular events with this insulin over its comparators,” according to a document posted on the Food and Drug Administration’s website.

The FDA document did note that degludec has been associated with lower risk of hypoglycemia, or dangerously low blood sugar, and said that factor “will be an important consideration in the overall benefit-risk assessment of this product.”

The news stokes concerns that degludec’s benefits may not outweigh its potential risks in the eyes of U.S. regulators, who have already delayed a decision on the product.

Any setback for degludec - which Novo plans to market under the brand name Tresiba - would be good news for rivals Sanofi and Eli Lilly.

“Given the known concerns of the FDA towards cardiovascular risk from new diabetes drugs, today’s statement is likely to create volatility in the Novo Nordisk share price,” Deutsche Bank analyst Tim Race said in a research note.

“Based on published data to date we expect a positive panel vote and eventual approval of Tresiba. However, we have not seen all the safety analysis and as such the risk remains that the FDA could require further analysis or clinical studies in order to gain confidence in Tresiba’s profile,” Race added.

Degludec was recommended for approval by the European Medicines Agency last week, but the U.S. decision is seen as pivotal, since this is where the bulk of future sales is expected to be generated.

Degludec is central to Novo’s aim of ending the dominance of Sanofi’s Lantus, the current market leader in the long-acting insulin market. Eli Lilly is also developing a rival drug that is a few years behind.

Novo is the world’s largest insulin maker and a key player in treating diabetes, a disease that is on the rise worldwide.

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Reuters