John Stamos Defends Himself Against Allegations of Drug Use and Underage SexMARQUETTE, Mich. -- Actor John Stamos testified Tuesday he did not use cocaine or behave inappropriately six years ago with an underage girl now accused of trying to squeeze $680,000 from him by threatening to sell compromising photos to celebrity tabloids.

Testifying in U.S. District Court, the 'Full House' and 'ER' star acknowledged having a "flirty" e-mail relationship with defendant Allison Coss after socializing with her in 2004, but denied knowing she was 17 at the time.

Stamos, 46, also denied defense lawyer Sarah Henderson's accusation that he stripped and sat in a hot tub in his hotel room with Coss, who was wearing only underclothes. And he insisted knowing all along that no photos of him using illegal drugs existed, despite corresponding online with someone claiming to have such pictures.

Coss, now 24, and 31-year-old Scott Sippola are charged with conspiracy and two counts of extortion. If convicted, the Marquette couple could get up to five years in prison on the conspiracy charge and two years on each extortion count.