Eli Lilly & Co. said its fourth-quarter profit declined 12% as the drug maker entered a new round of patent expirations that will erode sales of top drugs throughout 2014.

The Indianapolis company lost U.S. patent protection for its No. 1 product, the antidepressant Cymbalta, in December, which triggered the introduction of competing low-cost generic copies. Lilly is due to lose patent protection for bone-building drug Evista in March, which will further contribute to sales and profit declines for 2014.

Lilly has submitted four new drugs for regulatory review—one for gastric cancer and three for diabetes—and is hoping approvals of those will help it return the company sales and profit growth after this year.

"We're under no illusions," Chief Executive John C. Lechleiter said in an interview. "We need to get back to growth by launching new products." He said Lilly intends to pursue acquisitions to further expand its animal-health business, including deals that widen its presence in animal vaccines and in emerging markets.

The company continues to experience R&D setbacks. On Thursday, Lilly said it no longer plans to seek regulatory approval for liprotamase, an enzyme-replacement therapy for people with a pancreatic disorder. U.S. regulators had declined to approve the drug in 2011, asking for more clinical trial results.

Lilly also is trying to maximize prospects for its older products, in some cases by raising prices but in other cases by offering rebates and discounts to certain payers in exchange for their business. Effective Jan. 1, U.S. pharmacy-benefits manager Express Scripts Holding Co. made Lilly's Humalog the exclusive rapid-acting insulin available to members with diabetes, replacing a competing product from Novo Nordisk A/S of Denmark.

Dr. Lechleiter said Lilly offers discounts to secure such contracts, but the trade-off is that Humalog is likely to be used by more people because of its exclusive position in Express Scripts's list of preferred drugs.

Separately, Sanofi SA filed a patent-infringement lawsuit against Lilly in U.S. district court in Delaware Thursday, alleging that one of Lilly's proposed new insulin products would infringe upon patents covering Sanofi's Lantus insulin. Lilly has said previously that its proposed product, known as insulin glargine, is similar to Lantus.

Lilly and its partner Boehringer Ingelheim, recently filed for U.S. regulatory approval of their insulin glargine product.

According to Sanofi, Lilly challenged the validity of several patents for Lantus but said it would not launch its product before the expiration of Sanofi's patent on the active ingredient in Lantus, which is in force through February 12, 2015.

A Lilly spokesman couldn't immediately be reached about the Sanofi lawsuit.

For the fourth quarter of 2013, Lilly earned $727.5 million, or 67 cents a share, down from $827.2 million, or 74 cents a share, a year earlier. Excluding charges for restructuring and an acquisition and other items in the latest quarter, earnings would have been 74 cents a share, matching the mean estimate of analysts surveyed by Thomson Reuters.

Fourth-quarter sales declined 2% to $5.81 billion but exceeded the Thomson estimate of $5.46 billion. U.S. revenue dropped 6%, while revenue outside the U.S. rose 1%.

Sales of Cymbalta declined 38% to $883.2 million on the loss of patent protection in the U.S. But sales of several other drugs increased and were above analysts' expectations, including Humalog and erectile-dysfunction treatment Cialis.

Lilly's animal-health division posted sales of $578.4 million, up 4% from a year earlier. Lilly has relied on growth on its animal-health division to help soften the impact of patent expirations in its core human-health business.

For full-year 2013, Lilly's profit rose 15% to $4.68 billion, or $4.32 a share, on revenue growth of 2% to $23.1 billion. Excluding certain items, earnings for 2013 were $4.15 a share.

For full-year 2014, Lilly expects earnings to decline to a range of $2.77 to $2.85 a share, on revenue of $19.2 billion to $19.8 billion.

—Ben Fox Rubin contributed to this article.

Write to Peter Loftus at peter.loftus@wsj.com