The 38-year-old -- who played 63 Tests in which he took 226 wickets -- says in his autobiography 'Speed Demons', which is being serialised in the 'Daily Mirror', he had gone in 2004 to see a psychologist about his 'demons'.
This was at a time when he was considered the world's best bowler -- Australian spin legend Shane Warne has listed him as being one of the top 50 cricketers of all time.
Harmison -- who is one of a series of England cricketing greats such as opener Marcus Trescothick and all-rounder Andrew Flintoff to have spoken about suffering from depression -- said he suffered from depression on long tours abroad but found it pursued him even in a home series.
"It was the same story -- only worse. I was in no position to celebrate. I was in no state to do anything.
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"England won all seven Tests but as that summer went on I could feel the brightness growing darker.
"The horrible truth was those same feelings, which had consumed me on trips abroad, were overpowering me again -- and this time it had nothing to do with being away from home.
"The demons had not bothered to travel. They had come to get me at home, in the middle of a very successful English summer.
"The honest answer was "Maybe". I can't say there haven't been dark times where I thought it would be easier if I wasn't here.
"It was clear I was clinically depressed and medication would be the way forward. I've been on it ever since."
Harmison, who took 17 wickets in the extraordinary Ashes series win over Australia in England in 2005, is less sympathetic towards Jonathan Trott judging the language the England batsman used to describe his reason for returning early form the 2013/14 tour of Australia as not ringing true.
"When it comes to Jonathan Trott's illness, it's not that I question it, but I'd like to know deep down what the problem is," said Harmison.
"The statements he's made, the things he's said, the language he's used, makes me wonder if he just pulled the ladder up because the game got too hard.
"When he came back early from Australia after being bombarded by Mitchell Johnson and used the word "nutcase" while talking about what happened, I found that puzzling.
"When I saw Trott interviewed after coming home, I thought 'Wow, he's not poorly, he's weak'. He was describing someone who was mentally not very strong.
"I hope it was a mistranslation, but I have a nagging doubt Trott left the tour because he thought it was tough rather than because he was ill.