Pfizer Inc. and Microsoft Corp. said on Thursday they filed parallel lawsuits against Web site operators and spam advertisers that sell illegal versions of Pfizer's Viagra.

The companies said the lawsuits follow a seven-month investigation to discover the identity of two Web site operators together with those advertising them via spam e-mails.

Pfizer has filed suit against CanadianPharmacy and E-Pharmacy Direct, while Microsoft has filed civil actions against the spam advertisers for the Web sites. Microsoft has also filed three suits against spam advertisers who advertise unauthorized or counterfeit versions of Viagra on other online pharmacies under such names as Discount RX, Virtual RX, and EzyDrugStore.com.

With Microsoft going after illegal spammers and Pfizer taking on online sellers of illegal medicines, they said they are "targeting the entire supply chain."

Pfizer general counsel Beth Levine said there was no accurate way of calculating how much the company is losing in Viagra sales as a result of illegal Internet sales of the best-selling erectile dysfunction treatment.

The pharmacy spam rings have allegedly sent hundreds of millions of e-mail messages to Microsoft's MSN Hotmail customers within the past year alone, Microsoft said.

The identities of the individuals controlling the sites are unknown, the companies said.

But Aaron Kornblum, Microsoft's Internet safety enforcement attorney, said now that the lawsuits are active they have subpoena power to go to Web hosting services, payment processors and shipping distributors to try to track down the culprits.