"I did make a couple of bad choices, but that was a long, long time ago," Andreu was quoted as saying in the newspaper.
"It's not something to be proud of. I did use EPO, but only for a couple of races. I tried my best never to use performance-enhancing drugs."
The newspaper reported that the other rider refused to give his name, as he was still involved in the sport.
Although both riders confirmed they never saw Armstrong taking banned substances, Andreu testified in 2005 along with his wife Betsy that Amstrong admitted to using EPO, growth hormones, testosterone and cortisone. The Texan is said to have received these in October 1996 from a doctor of the Indiana University Hospital.
"There¹s always going to be the guy who denies and denies that he¹s ever used something," said Andreu.
"Nobody really knows what that guy is really doing when he goes home and closes the door."
The seven-time Tour de France winner Armstrong has never tested positive for performance-enhancing drugs and denies all allegations made against him.
The revelations are just the latest in a line of drugs scandals to hit Armstrong's former lieutenants.
Tour de France winner Floyd Landis tested positive for testosterone after stage 17 of this year's Tour, while Roberto Heras and Tyler Hamilton were both found guilty of blood doping and handed bans.