AJ Allmendinger said he tested positive for a prescription drug called Adderall, causing his indefinite suspension from NASCAR.
Speaking openly for the first time since being suspended, the 30-year-old told FOX Sports that he naively took an "energy pill" that was given to him by a friend when he got to Kentucky for a Cup race, as he was feeling tired that day.
On June 29, Allmendinger was tested randomly by NASCAR and the results caused his immediate temporary suspension, which later turned into indefinite when a 'B' sample also tested positive.
The ingredients found in Adderall, a prescription drug used to treat attention deficit hyperactivity disorder (ADHD) - which Allmendinger doesn't claim to suffer from - include amphetamines, a stimulant banned by NASCAR's substance abuse policy.
He said that following the positive tests he found out the pill he had taken was effectively prescription Adderall.
"I got to Kentucky early for a few sponsor commitments and went to hang out with a buddy for a couple of days before we got going," Allmendinger said.
"We were out early evening. I hadn't been sleeping well — all season, really with the way things had been going, obviously, the expectations and everything like that. I was really tired, had no energy, nothing. We were out, he had a couple of his friends with him and I was struggling to even stay awake. One of his friends said, 'Oh, I have an energy pill that I take for working out.'
"I didn't think anything of it because I've taken energy supplements for working out, that my trainer gives me. So I didn't even think about it. That was my big mistake. It was nothing crazy. It just gave me a little more energy."
Allmendinger said that his struggles on the track during the season had him under massive pressure and not enjoying his biggest break in NASCAR after he fulfilled a dream of driving for Roger Penske.
He also said he will continue to fight to get back in NASCAR after joining the 'Road to Recovery' program, designed by the ruling body to give suspended drivers a route to be reinstated.
"I was really struggling mentally trying to figure out what would make it fun again, if I really wanted to do this," Allmendinger said. "Is this the right way to do this? In this time of being away from racing, it made me realize I love racing. I love being a race car driver. I love being in NASCAR but, more importantly, being able to compete each weekend. That's what I want to do.
"Sometimes you need time to figure out these things. Now something that I've worked for for so long has been taken away from me and I have to be home on weekends. I'm not in a race shop or getting on an airplane to go to a racetrack. That's really hard. It's shown me how much I really love this sport and I'll do whatever it takes to get back."