The company's latest report states that this increase, which represents a Compound Annual Growth Rate (CAGR) of 5.3% and relates to eight major markets (the US, the UK, France, Germany, Spain, Italy, Japan and Canada), will occur fastest in Japan, due to its high ADHD prevalence.

Despite the stigma attached to mental health conditions, this country will witness a more impressive ADHD treatment market CAGR of 15.7%.

Mr Dale Prior, analyst for GBI Research, says: "ADHD prevalence will rise steadily across the major markets, from approximately 15 million cases in 2013 to just over 18 million by 2020. However, diagnosis rates are particularly low outside of the US, as the disorder is still treated with considerable skepticism.

"Debate remains about the fear of under- or over-diagnosis, the causes and legitimacy of ADHD as a condition, and the skew towards greater prevalence in the US. Research has suggested that ADHD is just as prevalent in other countries as in the US, but it is significantly more recognized in this market."

Mr Prior notes that this trend is expected to change over the coming decade, due largely to an increased recognition of adult ADHD.

The fifth edition of The Diagnostic and Statistical Manual of Mental Disorders, which was published in 2013, broadened the diagnostic criteria for ADHD, recognizing that impairment from symptoms of the disorder may develop in later life.

He comments: "These developments herald the final acceptance of adult ADHD in a clinical setting and mean that many people who met the 'partial remission' criteria will now meet the full criteria, leading to an increase in diagnosis.

"However, the continuing reluctance of many patients to seek treatment for mental health conditions will be a barrier to ADHD diagnosis rates, and consequently to any further market growth."