Plenty, according to the US Patent and Trademark Office, which partially rejected the patent on Pfizer’s little blue impotence pill after deciding a chemical ingredient in the herb is a similar type of enzyme inhibitor found in Viagra, Bloomberg News reports.

A PTO appeals board last Friday upheld a decision that an element, or claim, of the patent for a method of treating impotence didn’t cover a new invention. The patent claim was key to an infringement suit Pfizer filed in 2002 against Lilly over its rival Cialis drug. The decision, posted on the PTO’s Web site, can be appealed, Bloomberg continues. The patent expires in 2019, and Pfizer has another patent on Viagra that expires in 2012.

A Pfizer spokesman tells Bloomberg other elements of the patent remain valid and the decision has “no effect” on Pfizer’s claims relating to Viagra. The drugmaker has until April 12 to seek a rehearing from the PTO or appeal to the U.S. Court of Appeals for the Federal Circuit in Washington, according to Bloomberg, which adds the PTO will cancel that element of the patent if Pfizer doesn’t act (see letter from attorney).

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. Pfizer argued there was no evidence the herb treats impotence and treatments using Yin Yang Huo 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. A study issued in 2008 compared Horny Goat Weed with Viagra, and found it may have fewer side effects but not may be as beneficial.