U.S. Sen. Elizabeth Warren has asked the Federal Trade Commission "closely scrutinize" a $16.5 billion deal in which Novo Nordisk's parent foundation would acquire Catalent, a leading contract drug manufacturer, over concerns the acquisition would give the pharmaceutical company an unfair advantage in the burgeoning market for drugs used to treat obesity and diabetes.

In a letter dated Wednesday, Warren noted that Catalent fills and packages syringes and injection pens for Novo Nordisk and other drug manufacturers, including its biggest rival, Eli Lilly, which sells competing medications and also contracts with Catalent to fill and package its drugs. She noted that Eli Lilly has described Catalent as an "integral" player in its manufacturing of diabetes and obesity drugs.

"I am concerned that Novo Nordisk's merger with Catalent will give Novo Nordisk unprecedented visibility into and control over its competitor's production capacity, costs, and business practices, and the ability to preference its own products and obstruct its competitors' use of Catalent to produce" the drugs, she wrote. The drugs belong to a class of medicines known as GLP-1s.