In a lawsuit filed in Los Angeles, the non-profit group attacks Pfizer's commercials for Viagra as false and misleading and claims adverts featuring younger men have encouraged recreational use of the prescription drug.

Research has found Viagra is often used illegally in conjunction with crystal methamphetamine to form a party drug "cocktail."

While crystal meth can heighten sexual desire, it also can impair the ability to have an erection, said Michael Weinstein, president of the non-profit group. "In order to satisfy that heightened desire, you have to take Viagra."

The foundation, the largest provider of AIDS healthcare, prevention and education in the US, cited studies linking Viagra with increased risk of HIV and sexually transmitted disease infection, both on its own and in combination with crystal meth, which increases libido but can cause impotence.

It quoted from a review in The American Journal of Medicine which analysed data from 14 studies related to Viagra use and concluded that: "most studies reported frequent sildenafil (Viagra) use in men who have sex with men, and several showed independent associations between use of the drug and sexual risk behaviour, as well as an increased risk for STDs, including HIV infection."

Pfizer has denied it promotes the recreational use of its best-selling drug.

The suit seeks to halt the New York company from running adverts such as those used to promote the drug's use on New Year's Eve and Super Bowl Sunday, which include taglines such as "Be this Sunday's MVP" (most valuable player), said Tom Myers, of the AIDS group.

The suit also seeks to force Pfizer to undertake a public information campaign on the dangers of misusing and abusing the drug. And it seeks an unspecified amount to cover an increase in treatment costs at the group's free clinics.

Pfizer said it already supported AIDS prevention efforts, including a three-year, six million dollar project undertaken in 2003 in nine southern states.

The advertisements in question featured younger-looking men than earlier Viagra adverts, which used retired Sen. Bob Dole, then in his 70s. Mr Myers said the newer adverts implied the drug was meant to enhance the sexual experience and not to treat a medical condition.

A Pfizer official warned against confusing age with the degree of impotence.

In 2004, the Food and Drug Administration warned Pfizer that some television adverts for Viagra failed to note it was to treat impotence or provide information on its major side effects.

Filing of the suit came as Pfizer announced it would cut 10,000 jobs in seeking to trim its annual costs by 2 billion dollars.