Pfizer Inc.’s patent on its impotence drug Viagra has been partially rejected after the U.S. Patent and Trademark Office said it wasn’t different enough from a Chinese herb known as Horny Goat Weed.

An appeals board within the agency on Feb. 12 upheld a decision that an element, or claim, of the patent for a method of treating male erectile dysfunction didn’t cover a new invention. The patent claim was key to a patent-infringement suit Pfizer had filed in 2002 against Indianapolis-based Eli Lilly and Co. over its rival Cialis drug. The decision, posted on the PTO’s Web site, can be appealed.

Chris Loder, a Pfizer spokesman, said other elements of the patent remain valid, and the decision “has no effect on Pfizer’s claims relating to Viagra.” He declined to comment on how the decision would affect the suit against Indianapolis- based Lilly.

The lawsuit, in which Pfizer claimed Lilly was infringing the patent, had been put on hold while the patent office review was under way. Bayer AG, which makes the impotence pill Levitra, had also requested the agency take a second look at the patent.

New York-based Pfizer has until April 12 to seek a rehearing from the PTO’s Board of Patent Appeals and Interferences, or appeal to the U.S. Court of Appeals for the Federal Circuit in Washington, which specializes in patent law, according to a letter Lilly lawyer Richard K. Herrmann filed yesterday with the federal court in Wilmington, Delaware.

If Pfizer takes no action, the PTO will cancel that element of the patent, Herrmann said in the letter addressed to U.S. District Judge Joseph J. Farnan. The lawyer said the rejected claim was the only one at issue in the litigation.

In its decision, the board said the patent claim was the next logical step up from using the herb Yin Yang Huo, known as Horny Goat Weed, to treat impotence. A chemical ingredient of the herb is a similar type of enzyme inhibitor that’s in Viagra, the board found.

Pfizer had argued there was no evidence the herb treats erectile dysfunction. It also said that treatments using Yin Yang Huo also required “yellow rice wine, genital massage, rest, bathing in an herbal mixture and abstinence from intercourse and therefore does not establish that the treatment effect was due to Yin Yang Huo alone,” according to the board’s report.

The patent expires in 2019, and Pfizer has another patent on Viagra that expires in 2012, according to information on the U.S. Food and Drug Administration’s Web site.

Mark Taylor, a spokesman for Lilly, didn’t immediately return messages seeking comment.