A similar claim is made by UK mental health charity Mind:
1 in 4 people will experience a mental health problem of some kind each year in England.
They cite an NHS survey from 2007, which includes a more specific figure in its summary:
in 2007 nearly one person in four (23.0 per cent) in England had at least one psychiatric disorder and 7.2 per cent had two or more disorders
One of the attachments on that page is a detailed report of the findings titled "Adult psychiatric morbidity in England, 2007: Results of a household survey. Edited by Sally McManus, Howard Meltzer, Traolach Brugha, Paul Bebbington, Rachel Jenkins"
Section 12, "Psychiatric comorbidity", includes the statistic:
Just under a quarter of adults (23.0%) met the criteria (or screened positive) for at least one of the conditions under study.
From the detailed tables in that section, the conditions covered are:
- Generalised anxiety disorder (GAD)
- Mixed anxiety/depressive Disorder
- Obsessive compulsive disorder (OCD)
- Depressive episode
- Panic disorder or any phobia
- Alcohol dependency
- Drug dependency
- Borderline personality disorder (BPD)
- Antisocial personality disorder (ASPD)
- Post-traumatic stress disorder (PTSD)
- Attention deficit hyperactivity disorder (ADHD)
- Eating disorder
- Problem gambling
- Suicide attempt in past year
There are specific definitions included for each of these; for instance:
Drug misuse is defined by the World Health Organisation as the use of a substance for a purpose not consistent with legal or medical guidelines, for example the non-medical use of prescription medications or the recreational use of illegal drugs.
Note that nicotine addiction is not covered by this section, so somebody who smokes regularly but does not use any other drugs would not be included in the 23% figure.
Similarly, the appendixes explain exactly how "Mixed anxiety/depressive Disorder" and "Depressive episode" were diagnosed, based on a standardised survey called "Clinical Interview Schedule (CIS-R)".
It would of course be possible to pick a slightly different set of conditions, and use different definitions or thresholds, and come to a different number.
It is also worth noting that this statistic is not the main result of this survey, merely a striking figure which has been picked up on. The survey's main aims were to measure correlations, both between the different conditions, and against other factors such as age, sex, and social background.