NEW YORK -- Pfizer has ended its research on whether Viagra can be used to treat female sexual problems.

The company said tests using the anti-impotency drug, which began in 1996 and involved about 3,000 women, were inconclusive.

"Diagnosing (female sexual arousal disorder) involves assessing physical, emotional and relationship factors, and these complex and interdependent factors make measuring a medicine's effect very difficult," said Dr. Joe Feczko, president of Worldwide Development at Pfizer.

At least 10 drug companies have been trying to develop a female equivalent for Viagra, which has been prescribed to 23 million men since it was launched in 1998.

Karen Katen, president of Pfizer Global Pharmaceuticals, said while the company is disappointed the program wasn't a success, that's the nature of drug development.