Jackson disclosed taking the injections to fightersonlymag.com in an interview published Wednesday. He said he had sought medical attention after suffering a knee injury before the fight, which he lost by unanimous decision.
"I almost pulled out, but then I went to see the doctor and he told me to talk to an age-management doctor," Jackson told the website. "They tested me and said my testosterone was low; they prescribed me testosterone, to bring my testosterone levels back up ... so that I am the same as young people, like when I was 25, and it would help build my knee up.
"I hurt my knee like a month ago and I only did three shots of testosterone, but it put a lot of weight on me, a lot of muscle on me, but it healed my knee up good enough to where I could fight," he said, according to the report.
"It was hard for me to train, it takes time to heal, I couldn't do certain things, but this was my first time ever using testosterone," Jackson said. "I took what the doctor prescribed to me and I went to the pharmacy ... I gave myself small doses and that (stuff) immediately changed me. That's why I am saying now I am not going to retire."
UFC co-owner Lorenzo Fertitta, UFC president Dana White and Jackson could not immediately be reached for comment by ESPN.com.
The 33-year-old Jackson has lost two fights in a row and three of his five most recent bouts. He has a professional mixed martial arts record of 32-10.
Jackson was originally scheduled to compete Jan. 28 in Chicago, but the former Pride champion persuaded UFC officials to allow him on the card in Japan, where he had not fought since making his UFC debut in February 2007.
However, he failed to make the required 206-pound limit for a nontitle light heavyweight bout. Jackson tipped the scale at 211 and forfeited 20 percent of his fight purse.
He first made mention of suffering an injury during a postfight interview with Fuel TV. During that interview, Jackson did not disclose the nature of his injury or mention he had received testosterone injections.
"I really wanted to fight here (in Japan)," Jackson said at the time. "You guys have no idea. I've had ups and downs in my camp. Me and Dana White almost bumped heads again but he understood how much I wanted to be here."
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.