In its complaint, the FTC said Amgen would be able to use the powerful position it has with insurance companies and pharmacy benefit managers due to its blockbuster drugs to pressure them into favorable terms for Horizon's two key products - the fast-growing thyroid eye disease treatment Tepezza and gout drug Krystexxa.

FTC Bureau of Competition Director Holly Vedova called the lawsuit "the FTC's first challenge to a pharmaceutical merger in recent memory."

"Rampant consolidation in the pharmaceutical industry has given powerful companies a pass to exorbitantly hike prescription drug prices, deny patients access to more affordable generics, and hamstring innovation in life-saving markets," she added.

Amgen said in a statement it was disappointed by the FTC decision and said it believed that it had "overwhelmingly demonstrated" that the deal had no legitimate competitive issues. "We ... intend to work with the court on a schedule that would allow the transaction to close by mid-December," the California-based biotech said.

Horizon shares were down 15% to $94.98 in New York trading on Tuesday.

Amgen sees revenue from the Horizon medicines as a bulwark against competition that has eroded sales of its blockbuster rheumatoid arthritis drug Enbrel. Other key drugs in Amgen's product portfolio, such as psoriasis therapy Otezla, face the loss of patent protections over the next few years.

Jefferies analyst Michael Yee said drugmakers may change how they view M&A targets based on this FTC case and would emphasize buying smaller companies or those with products still in clinical trials rather than established in the market.

The FTC, which currently has three Democratic commissioners, voted 3-0 to approve the challenge.

In recent years, the agency has typically identified therapeutic overlaps in the companies involved in deals and required one of the medicines to be divested.