Dax Shepard has relapsed in his 16-year sobriety journey.

The 45-year-old actor has been battling an addiction to prescription medication for over a decade, and has confessed that following his recent motorcycle accident, he relapsed into taking Vicodin again after several years sober.

Introducing the topic on his 'Armchair Expert' podcast, he said: ''I have a tremendous amount of fear about doing this. Maybe some people will feel a sense of betrayal because we preach honesty and I was being dishonest.''

Dax went on to say that although his August accident did play a part in his relapse, he has been battling with relapses since 2012.

He continued: ''Eight years into sobriety, I had not done a single shady thing. In 2012, my father was dying. He got diagnosed with cancer in August and then he died December 31. I was going back [and forth] nonstop to do all the treatments with him and take him to chemo ...

''I also got into a motorcycle accident going to work on 'Parenthood' [at the time]. I immediately called my sponsor, and I said, 'I'm in a ton of pain and I gotta work all day. And we have friends that have Vicodin.' ''

The actor's sponsor agreed to let him get a prescription for Vicodin from his doctor, and the medication was given to his wife, Kristen Bell, who would make sure he never took more than he was supposed to.

However, during one trip to visit his father, he left his pills at home because Kristin wasn't travelling with him, and ended up taking some of his father's Perocet instead.

He added: ''I go, 'I have a prescription for this and I was in a motorcycle accident. I'm going to take some too.' You know, we had so little in common and so much friction. But the number one thing we had in common was we were both f****** addicts and we had never used anything together. And we sat there stoned and looked at the lake. And in that moment, I felt elation and I was just happy. ... I probably took twice [as much] of what my other prescription was.''

After taking the pills, Dax was left feeling ''overwhelmed'' and broke down in tears to Kristen, who had made a surprise visit to see her father-in-law.

And eight years later, after his most recent motorcycle accident, Dax confessed he endured a similar struggle.

He said: ''That cycle happens maybe three or four more times,'' he recalled. ''I feel shady, but I don't feel like this is a problem. I didn't desire more when the thing was over.

''After I ride sometimes on the track, I feel I'm entitled to take two Vicodin at the end of the day because I am in pain. That again doesn't feel that crazy. [But] I'm lying to other people and I know I have to quit ... I start getting really scared, and I'm starting to feel really lonely. And I just have this enormous secret.''

Now, Dax is seven days clean.

He revealed: ''I've had a lot of friends that I've watched go through this whole cycle, and I finally have the humility to say I will not be any different. I won't be special. I won't be smarter. I will be exactly like everyone else.''