Viagra is one of the best-selling drugs in the world and one of the most famous brand names ever.

But a report in a new pharmaceutical journal says that by early 2006, Cialis - the rival drug developed by Bothell-based ICOS Corp. - could overtake Pfizer Inc.‘s Viagra as the most popular erectile dysfunction drug in the United States.

Fritz Krieger, editor of the Journal of Longitudinal Data, makes that prediction in an article for the publication’s first issue this month.

“Cialis appears to be the main threat to Viagra,” Krieger wrote in his article titled “E.D. Wars.”

“Its 36-hour active life,” he continued, “and the fact that it does not interact with high-fat foods or alcohol to the extent Viagra does, are both advantages bound to propel the drug to category leadership.”

Matt Beebe, leader of the Cialis brand team at Eli Lilly & Co. that is jointly manufacturing and marketing the drug, wouldn’t comment specifically on the journal report.

He said, however, that joint-venture partners Lilly and ICOS are pleased with the drug’s U.S. sales over the past nine months.

“Some of the comments (Krieger) made are very consistent with the comments we’re hearing back from doctors and patients,” Beebe said. “The feedback we’re receiving around the U.S. has been tremendously positive.”

Pfizer representatives could not be reached for comment.

Krieger, formerly an executive in the pharmaceutical industry, is using new detailed data from prescription-tracking companies in his Ohio-based journal. This “longitudinal” data can show how long patients stay on a particular drug and details about patients’ switching between brand-name drugs.

Using data gathered by Verispan, a prescription tracking service, Krieger found that Viagra has lost market share to Cialis and Levitra, made by Bayer and GlaxoSmithKline.

The numbers show Viagra has lost 19 percent of its market share since the two other drugs debuted in the United States last fall. The new drugs, conversely, had captured about 20 percent of a growing market as of June. Cialis accounted for more than half that total.

While Viagra still held more than 75 percent of the total market for erectile dysfunction drugs as of June, the percentage of new patients starting on Viagra had slipped to 60 percent. Cialis accounted for 21 percent of the new prescriptions.

Among those switching between the three drugs, patients were seven times more likely to change prescriptions to Cialis as they were to stop taking the drug.

Cialis gained nearly 178,000 users that way during the first half of the year, while losing 25,432.

Viagra, on the other hand, lost nearly 243,000 patients to Cialis and Levitra and gained just 38,412.

Combined with other aspects of the market, the news is bad for Viagra, Krieger wrote. Assuming the trends continue at the same rate, Viagra‘s market share could dip under 50 percent by summer 2005, and Cialis would surpass Viagra in the market by early 2006.

Krieger added that he produced his article independently for his new publication; he owns less than 500 shares each of both Pfizer and Lilly.

ICOS estimates worldwide sales of Cialis will total between $500 million and $600 million in 2004, the drug’s first full year on the market. Last year, Pfizer reported worldwide Viagra sales of $1.7 billion.