Mark Williams had suffered for years from symptoms that 'looked like depression'. Racing thoughts and confused states left him feeling miserable, says the father-of-one.
In 2011, he had a mental breakdown after his wife Michelle developed post-natal depression and he started to experience suicidal thoughts.
'I was having many more bad days than good days. I just didn't care about anything,' he says. 'My body felt inexplicably strange and started to appear as if it was shutting down. I just wanted to stay in bed.'
His GP drew the most obvious conclusion, diagnosing Mark with serious depression and putting him on the antidepressant drug, citalopram, at the maximum dose. 'I stayed on that dose for four years,' says Mark, a charity worker from Bridgend.
Psychiatrists diagnosed him with bipolar disorder - the condition formerly known as manic depression, which causes moods to swing between extreme highs and lows.
'I knew it wasn't that. The symptoms just didn't fit,' he says. 'I wasn't suffering from manic fantasies or anything like that.'
Mark persuaded his community mental health team to send him to another NHS psychiatrist. This time he was diagnosed with adult attention deficit hyperactivity disorder or adult ADHD.
'It made sense,' says Mark. 'It was as if all of the jigsaw pieces came together.'
Adults with ADHD may not have the hyperactivity seen in children with the condition, but they may find it hard to do everyday tasks such as remembering information, concentrating, organising tasks and following directions.
As Mark describes it: 'My mind is always going "got to do this, got to do that". Boredom is a massive problem. It is also difficult for me to focus on things I'm not immediately interested in.'
Adult ADHD is a comparatively new diagnosis - for years, mainstream psychologists believed childhood ADHD 'burnt itself out' in late adolescence.
Now it's thought the condition can persist into adulthood, with studies suggesting 60 per cent of children diagnosed with ADHD suffer symptoms in early adulthood, according to the Royal College of General Practitioners. Research is yet to confirm how many older adults have it.
ADHD in children is easier to detect as their symptoms of inattention, often in school, are more obvious.
The official NHS guidance states: 'Adult symptoms of ADHD tend to be far more subtle than childhood symptoms. Hyperactivity tends to decrease in adults, while inattentiveness tends to get worse as the pressure of adult life increases.'
The problem is that many adults affected by it are misdiagnosed, as Mark was, with depression - that's the suggestion from a new study of patients referred to a clinic with diagnoses of depressive mood disorders. Nearly one in three was found to have previously undetected adult ADHD.
The study, which was revealed last month at the Anxiety and Depression Association of America conference in Philadelphia, suggests adult patients diagnosed with so-called treatment-resistant depression (where they have failed to respond to at least one type of antidepressant) are most likely to be suffering instead from undetected ADHD.
The study, which examined 105 patients with an average age of 40 found those who had adult ADHD were also most likely to have been put on courses of SSRI (selective serotonin reuptake inhibitor), antidepressant drugs that boost serotonin levels, in failed attempts to tackle their symptoms.
'ADHD is new as a diagnosis made in adulthood, so when people present with symptoms of depression, doctors typically won't ask any further questions about their history or assess for ADHD,' says lead researcher Tia Sternat, a psychiatrist at the START Clinic for Mood and Anxiety Disorders in Toronto.
But depressed patients with adult attention deficit hyperactivity disorder don't typically respond to SSRIs.
'They come in saying "I'm not happy, I'm tired, I'm anxious, I'm having trouble with attention" and what you are seeing are the adult signs of ADHD coming through.'
Adult ADHD is only the latest in a long list of illnesses commonly misdiagnosed by doctors as depression.
These include Alzheimer's and Parkinson's, diabetes, premature menopause, multiple sclerosis, anaemia, an underactive thyroid gland, the gynaecological condition endometriosis and even heavy snoring (or sleep apnoea). Such conditions can all affect people's moods. And the consequences of misdiagnosis can be serious if not catastrophic.
Patients not only run the risk of being put on powerful antidepressants with potential side-effects such as loss of libido, but they also may miss out on essential treatment which, given promptly, can cure or alleviate their real illness.
Even brain tumours have been mistaken for depression. Last October, Melvyn Scarborough, an amateur referee, revealed he had been forced to take a year off officiating games after suffering chronic tiredness and lack of balance. Doctors put it down to feeling depressed.
It was only by chance - after being knocked out by a ball at a match - that the 55-year-old from Sheffield was given a brain scan, which revealed a fist-sized tumour.
Within days he'd undergone a life-saving 12-hour operation and is having to take a year off work as a construction site supervisor while he recovers.
'I was really suffering before the diagnosis,' he says. 'I was falling down, tripping over things and making silly mistakes. All the symptoms were there and I told the doctors everything - but it wasn't picked up. I feel a bitter about that.'
Alex Mitchell, professor of liaison psychiatry at the University of Leicester, has found that family doctors in urban areas are liable to over-diagnose one in six patients with depression.
More shockingly, Professor Mitchell's study, published in The Lancet, found GPs in rural areas were likely to make three times as many incorrect diagnoses of depression as correct diagnoses.
He drew from a review of 41 studies involving more than 50,000 patients. The study recommended that if GPs evaluated people who might have depression over two appointments instead of one, the diagnostic accuracy rate rises dramatically to 90 per cent.
Dr Liz England, clinical lead for mental health at the Royal College of General Practitioners, says the NHS protocol of 'stepped care' for GPs should help to ensure patients are not rushed into being diagnosed with depression.
Under guidelines, patients with depression symptoms 'should be '"stepped up" through different care levels so they aren't initially diagnosed and given drugs for depression', she says. Patients should first be tried with 'low- level' remedies, such as self-help books.
If that doesn't work, they should be referred to mental health professionals for psychological therapies such as CBT. If that doesn't work, the patient might be referred for antidepressants.
GPs are not given time to make in-depth consultations, says Dr England. 'Often, people don't just come in with depression, but they have other problems in their lives.'
Meanwhile, many patients with ADHD may not only be taking antidepressants inappropriately thanks to misdiagnosis, they may also be missing out on drug therapies that may help their condition.
As Tia Sternat explains, while adults with ADHD may often complain of an inability to feel pleasure in life - a symptom also seen commonly in depression - the brain mechanisms behind the problem are very different.
In patients with depression, the loss of enjoyment is believed to stem from insufficient levels of the 'feelgood' brain chemical serotonin. Hence the use of SSRI antidepressant drugs.
However, in adult ADHD, the depressive symptoms appear linked to the lack of a different 'feelgood' brain chemical - dopamine.
Thus a different class of drug is required, one that may help to increase levels of dopamine, such as atomoxetine.
Mark has been taken off the SSRI drug citalopram. Four weeks ago, he was put on the ADHD drug, atomoxetine, which is thought to boost dopamine.
'I have seen the improvement already with my ability to focus my thoughts. I wake up feeling relaxed and focused,' he says.
Mark also uses mindfulness meditation techniques to help cope with his racing thoughts.
He believes the diagnosis has also helped with his work for a charity he founded, Fathers Reaching Out, which aims to increase awareness and understanding of postnatal depression and anxiety in men.
While he is happy to have finally received a diagnosis that makes sense to him, he regrets what he feels to have been decades of lost opportunities.
'I wish that I had been given medication at school. My lack of focus there meant my work was poor and I had low self-esteem.
'I would fidget all the time, which would result in me being humiliated and caned by the teachers.'
Hopefully, delayed diagnosis of ADHD may be changing, according to Dr England.
'It is only recently that anyone is talking about ADHD in adults,' she says.
'The more we can talk about these things then the more resources can be directed towards them.'