U.S. antitrust regulators found no wrongdoing in a deal made four years ago between German drugmaker Bayer and a division of Teva Pharmaceuticals to end patent litigation over the popular Yasmin birth control pill.

The Federal Trade Commission, in letters dated Sept. 26 but posted on the agency's website this week, said it had reviewed the matter and said, "No further action is warranted by the commission at this time. Accordingly, the investigation has been closed."

The FTC had been looking at the deal made by Bayer and Barr Laboratories, which was purchased by Teva, in 2008 to settle what was then a three-year fight over whether Barr infringed on Bayer's "Yasmin" patent to make its own generic contraceptives.

Under the supply and licensing agreement, Barr paid Bayer a fixed percentage of its revenues for the contraceptive, Bayer said in its 2011 annual report.

The FTC had no comment beyond the letter. Spokespeople for Bayer and Teva did not immediately respond to requests for comment.

The FTC has battled what it calls "pay for delay" settlements for years with mixed success. In the deals, brand-name drug companies typically sue generic firms for infringement and then settle, with the generic firm agreeing to delay entry into the market.

The FTC has also pushed for legislation to ban the deals.

Bayer's main oral contraceptive brand Yasmin has also been subject to controversy over health risks. Bayer earlier this year agreed to pay a combined $400 million to settle almost one third of about 6,000 legal claims in the United States that Yasmin caused blood clots.

In April, the Food and Drug Administration added information to the labels on Bayer's contraceptives Yaz and Yasmin, as well as others, to show they may raise the risk of blood clots.

All common birth control pills increase a woman's chances of getting potentially fatal blood clots, but the FDA said the danger may be greater for more recent pills that contain the synthetic hormone drospirenone.

Bayer is one of the largest players in the $8 billion global market for hormonal birth-control pills.

Yaz, a reformulated version of Yasmin, remains one of the U.S. top-selling contraceptives. Bayer had $374 million in Yaz sales in 2010, according to data from IMS Health. Sales of Yaz have slipped in recent years, after Teva and Watson Pharmaceuticals launched generic versions.