LONDON (Reuters) - A Chinese herbal remedy called horny goat weed is a promising alternative to Viagra for impotent men, Italian researchers said on Monday.

The herb has long held a reputation as a natural aphrodisiac. The lab experiments, which did not look at whether the plant actually increases desire, could lead to new drugs to help men get erections, said Mario Dell'Agli, a researcher at the University of Milan, who led the study.

"This could be the natural Viagra," he said in a telephone interview. "The novelty is that we have synthesized a new molecule that one day may be able to replace Viagra."

Erectile dysfunction is a common condition worldwide, and drugs like Pfizer Inc's Viagra, Eli Lilly and Co's Cialis or tadalafil, and Bayer AG's Levitra or vardenafil, work by increasing blood flow to the genitals.

But the medicines, which inhibit an enzyme called phosphodiesterase type 5 that restricts blood flow around the body, including to the penis, can have side effects ranging from headaches, upset stomach and visual problems including blindness.

The Italian team looked for alternatives by studying a number of plants reputed to boost sexual performance.

After homing in on horny goat weed, the researchers modified a compound in the plant called icariin and found it blocked the erection-inhibiting enzyme as well as Viagra did.

Because the compound targets the enzyme more precisely, it may have fewer side effects than Viagra, known generically as sildenafil, Dell'Agli said.

Further tests in animals and humans are needed but the extract from the herb represents a potential new erectile dysfunction treatment with fewer side effects, Dell'Agli said.

"The compound icariin is present in the horny goat weed in large amounts and its activity against (the enzyme) is lower compared to Viagra," he said. "But the new molecule we synthesized from icariin is as good as Viagra against (the enzyme)."