Pfizer and Microsoft have filed parallel lawsuits against two international pharmacy rings hawking generic versions of the pharmaceutical giant's erectile dysfunction drug Viagra.

The companies said legal actions filed Thursday in New York and Washington stem from a seven-month investigation during which they identified and tracked down the two most prominent parties allegedly responsible for selling an imitation version of the popular drug, as well as the spammers advertising the drug.

In the two lawsuits Pfizer filed, the drug company accuses CanadianPharmacy and E-Pharmacy Direct with promoting and selling Viagra knock-offs that are unapproved by the U.S. Food and Drug Administration. For its part in the parallel lawsuits, Microsoft has filed cases against the spammers advertising for those companies.

In total, there are 17 lawsuits against the defendants allegedly involved in the selling and advertising the knock-off drug.

"The collaboration between Pfizer and Microsoft is another wake-up call to those who abuse the Internet for illegal purposes," Brad Smith, senior vice president and general counsel for Microsoft, said in a statement. "Leading businesses are teaming up, pooling resources and sharing investigative information to stop this illegal activity at the source. At Microsoft we are combating this problem every way we can, including legal action, to ensure our consumers are protected from fraud."

Microsoft claims pharmacy spam rings have allegedly sent hundreds of millions of e-mail messages to Microsoft's MSN Hotmail customers within the past year alone. They have also accused the defendants of using "illegal and deceptive e-mail techniques" that violate the federal CAN-SPAM Act of 2003 and other state and federal laws.

According to some industry estimates, Viagra and similar drugs are estimated to account for up to one out of four spam messages.

Both companies say their investigations turned up widespread scams, in which orders are filled with illegal, unregulated products shipped from India to the United States and delivered by an airfreight forwarding company based in the United States.

"As the maker of Viagra, Pfizer is committed to protecting the public from health and other risks associated with the illegal sale of fake and unapproved generic products claiming to be Viagra," Jeff Kindler, executive vice president and general counsel at Pfizer, said in a statement. "Pfizer is joining with Microsoft on these actions as part of our shared pledge to reduce the sale of these products and to fight the senders of unsolicited e-mail that overwhelm people's inboxes."

Microsoft also filed three suits against spammers advertising other online pharmacies known as Discount RX, Virtual RX and EzyDrugStore.com.