Those allegations brought a strong denial Friday from Armstrong and the legal counsel for his Radio Shack-sponsored team.
Landis, who had his 2006 Tour de France title taken away after he tested positive for drugs, had insisted for almost four years that he was innocent of the doping charges.
In a series of e-mail messages to cycling officials, Landis said he had first taken drugs in 2002. He also said Armstrong, a seven-time Tour de France winner, had used performance-enhancing substances.
French newspapers have alleged for years that Armstrong has taken drugs, but he has never failed a drug test.
"When no one in cycling capitulated to his numerous but persistent false threats, demands and rants, Floyd Landis publicly aired the false and incredible concoctions he has been privately making for years," said a statement issued by the legal representatives of Armstrong's team.
"While these types of repeated, tired and baseless accusations against Lance have been proven false in the past, it is quite regrettable that so many in cycling are now attacked by a bitter and scorned Landis who, quite simply, has zero credibility."
Landis also accused current U.S. road racing national champion George Hincapie, three-time Tour of California winner Levi Leipheimer and five-time U.S. time trial champ David Zabriskie of doping.
Armstrong dismissed the allegations.
"It's our word against his word. We like our word, we like our credibility," Armstrong said Thursday.
"I would remind everybody that this is a man who wrote a book for profit that has a completely different version. He took a lot of people's money for his defense. Now when it all runs out, his story changes."
Armstrong said it was unlikely he would pursue legal action against Landis.