McCarthy, who said an addiction to drugs left him in a rut, was suspended on Thursday after the disciplinary panel of the British Horseracing Authority was told at a hearing that he tested positive for benzoylecgonine, a metabolite of cocaine, in a urine sample at Chelmsford on Oct. 15 last year.
He gave a reading of 150,300 nanograms per millilitre, with the threshold for riding set at 150 ng/ml.
The 42-year-old said he was "in a really bad place" at that time and had tried to take his own life.
"I just got into a bit of a rut," McCarthy told the hearing. "Drinking all the time, using drugs, cocaine, and trying to make things better. Obviously it doesn't make things better.
"I'm in a lot better place now than I was before, I just want to get my head down and do what's right.
"We all make mistakes. I made a mistake, I regret it. I just have to look forward and work hard."
McCarthy is banned from racing until April 21, with his six-month suspension backdated to October 22.