Microsoft and pharmaceuticals giant Pfizer are filing lawsuits against spammers and websites that allegedly sell versions of the popular anti-impotence drug Viagra.

The companies announced that the legal action is the result of a seven-month investigation to track down the alleged drug distributors as well as the spammers advertising them.

Pfizer has filed civil actions against two sites - CanadianPharmacy (doing business as cndpharmacy.com) and E-Pharmacy Direct (doing business as myepharmacydirect.com) - that allegedly promote and sell products not approved by the US Food and Drug Administration. It has also filed 10 other domain name actions.

In conjunction with Pfizer's suits, Microsoft has filed civil actions against the spammers advertising for those websites, as well as three suits against spammers who advertise other online pharmacies.

Together, these pharmacy spam rings have allegedly sent hundreds of millions of email messages to MSN Hotmail customers within the past year alone, according to Microsoft.

"The collaboration between Pfizer and Microsoft is another wake-up call to those who abuse the internet for illegal purposes," said Brad Smith, senior vice president and general counsel at Microsoft.

"Leading businesses are teaming up, pooling resources and sharing investigative information to stop this illegal activity at the source."

Jeff Kindler, executive vice president and general counsel at Pfizer, added: "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 email that overwhelms people's inboxes."

According to some industry estimates, Viagra and similar drugs account for up to one in four spam messages.