In this study, estimates for the prevalence of MPH, DEX and ATM prescribing were calculated for the period, 2003 to 2008, using data from THIN database. It was observed that prevalence of prescribing increased over this time, with a ~2-fold increase for children and adolescents, and a 4–5-fold increase for adults. Incidence of prescribing showed similar patterns whereby the incidence of prescribing was greater in children and declined in adulthood.
School-age children and adolescents
Overall, prevalence increased over the study period from 4.83 to 9.18 per 1000 patients aged 6–12 years. The highest prevalence of prescribing in this study was to boys aged 6–12 years (15.32 per 1000 boys aged 6–12 years in 2008). Prescribing to male patients in this age category was higher than to female patients (6:1 in 2003; 5.5:1 in 2008). These findings are in line with figures reported in the literature, which report differences in prescribing between the genders ranging from a ratio of 2:1 to 9 . It is not known to what extent this is a true behavioural gender difference and how much is due to factors such as the under-diagnosis and under-reporting of the condition in females. Interestingly, the relative increase in prevalence over the study for children 6–12 years was slightly higher in female patients compared to male patients (2.1 compared with 1.9 times increase). A similar observation was reported by Cox et al., who reported that in the US from 2002 to 2005, the rate of growth of ADHD drug prescribing to females was double that of males .
Treatment prevalence for adolescents aged 13–17 years followed a similar pattern whereby the overall prevalence doubled over the study period. The driver of this increase was prescribing to adolescent girls (3.15 increase over the 6 year period) although actual prevalence was again higher in male patients.
To our knowledge, only one previous study has examined the use of these drugs in children in the UK . The authors reported on the incidence and prevalence of MPH and DEX in boys aged 5–14 years from 1996–2001. This study reported a prevalence of 5.3 per 1000 boys in 1999.
A study from the Netherlands used computerised pharmacy dispensing records to examine the prevalence and incidence of psychotropic medications in Dutch children from 1995 to 1999. The highest prevalence was seen in children aged 5–9 years which, in 1999, was 13.9 per 1000 children . More recently, a retrospective analysis was conducted by Hodgkins and colleagues to estimate the incidence and prevalence of children, aged 6–17 years, receiving initial pharmacotherapy for ADHD between 2000 and 2007 from a large sample representative of the general population of the Netherlands . Data extrapolated from the PHARMO database to the Netherlands population demonstrated an increase in yearly incidence from 30 per 10,000 in the year 2000 to 75 per 10,000 in the year 2007. Prevalence increased from 110 per 10,000 in 2000 to 210 per 10,000 in 2007 .
A study examining prescribing trends for stimulants from 1992 to 1998 using North Carolina Medicaid prescription claim files reported an increase in prevalence from 44 per 1000 patients in 1992 to 95 per 1000 patients in 1998 in children aged 6–14 years. The authors of this paper acknowledged that the rates observed in their study were much higher than other studies reported; however they do not speculate as to why this is the case .
More recent studies from the US include a study by Zuvekas et al. who used the Medical Expenditure Panel Survey database to report prevalence of stimulant use from 1997 to 2002 in children aged less than 19 years . The prevalence increased from 27 per 1000 patients (95% CI: 23–31 per 1000) in 1997 to 29 per 1000 patients (95% CI: 25–33 per 1000) in 2002. They also reported the highest use of stimulants in children aged 6–12 years. Cox et al. used ambulatory prescription claims data of children aged 5–19 years from 2002 to 2005 and over this period reported a growth in prevalence of ADHD medications of 40.4% .
These utilisation studies suggest that especially in the US, the prevalence of stimulant use increased substantially during the last decade.
The current study has demonstrated a trend of increasing prevalence of pharmacological treatments in the UK, throughout the study period; however the highest prevalence figure reported of 15.4 per 1000 male patients aged 6–12 years is in line with or below those reported in both the Netherlands and the US.
More importantly this figure is also lower than the global prevalence of ADHD in children or that of hyperkinetic disorders in the UK, which were recently estimated to be 5% and 1.5% respectively [2, 27]. This is relevant as current NICE clinical guidelines recommend that for school-age children and young people with severe ADHD (hyperkinetic disorder), drug treatment should be offered as the first-line treatment and that medication will also be appropriate for patients with moderate levels of impairment who have refused non-drug interventions, or whose symptoms have not responded sufficiently to parent-training/education programmes or group psychological treatment .
Population surveys in adult populations estimate the prevalence of ADHD in adults to be between 2.5 and 4% [4, 8–10]. Whilst not all patients will require pharmacological intervention, NICE recommends that it should be the first-line treatment unless the person would prefer a psychological approach . The results of this study suggest a trend of increasing prevalence of prescribing of ADHD drugs to adult patients; however the numbers remain much lower than the estimated prevalence of the condition. There may be several reasons for this including that earlier NICE guidelines in 2000  indicated that medication should be tailed off in adolescence and the lack of licensed medicines for the treatment of ADHD in adulthood. It is expected that with the recommendation in the current NICE guidelines  for pharmacological treatment of adult ADHD, the increase in prevalence seen in this study will increase further and this may better reflect the prevalence of the disorder in a few years' time.
Strengths and limitations
A significant strength of the study was the use of a large database such as THIN which provided primary care data on a cohort of over 4500 patients. THIN has been used widely in epidemiological research, including studies on mental health [28, 29]. The use of THIN data allowed us to capture what is actually happening under normal conditions of clinical practice, rather than in selected samples of patients recruited into clinical trials. NICE guidelines recommend that although medications should be initiated by healthcare professionals with appropriate expertise in ADHD, GPs may continue prescribing and monitoring of medications; thus the use of a general practice database is a suitable data source for identifying and examining ADHD prescribing patterns. Nevertheless, these data might underestimate the overall prescribing rates in the UK since in some regions of the country specialist mental health teams or paediatricians remain the main prescribers and some GP practices will not prescribe medications for ADHD. This might particularly influence the estimates for prescribing to adults, since prescribing practice for those over the age of 18 years is still not well established in the UK and prescribing by specialists rather than GPs is still the norm in many regions.
A limitation of the data is that detailed information on the diagnoses was not readily accessible; therefore it was not possible to determine the severity of ADHD in the patients identified. An inclusion criterion for the study was that patients were required to be registered on the database during the study period 2003–2008 and have a minimum of one year of registration on the database. However, patients may have registered on the database at various points during this period or before this period. Therefore, the amount of follow-up time for patients registered later on the database may have been less than those registered earlier in the study period. For incidence calculations, patients prescribed ADHD drugs during this 12-month screening period were not included in the risk pool for subsequent years of the study. This resulted in varying look-back periods, in that in 2003, patients incident in 2002 were removed, whereas in 2008, six years of incident patients were removed. However, as the numbers of incident patients are small relative to the denominator, it is unlikely that this would influence reported rates significantly. A potential bias in the data when comparing prevalence between 2003 and 2008 is that some patients may contribute data in both years. Data from such patients would not contribute to the change in prevalence. This potential overlap in the cohorts was not taken into account when comparing the prevalence between 2003 and 2008.
The issue of over-prescribing of ADHD medicines was outside the remit of this study, as studies which have looked at this question have identified patients with an ADHD diagnosis who do and do not receive medication and/or psychological treatments, along with patients who receive medication but who do not meet the diagnostic criteria for ADHD [30, 31]. The detailed specialist records required to examine this are not routinely available for all patients on a general practice database such as THIN and so the current study cannot answer the question as to whether the stimulants (MPH, DEX and ATM) are over-prescribed. However, despite the increase in prescribing observed over the study period, the difference between the prevalence of the condition reported in the literature and the prevalence in this study of prescribing of these drugs to those with ADHD does provide some assurance that it is unlikely that these drugs are over-used in the UK. ADHD pharmacological treatments are generally accepted to have a favourable risk/benefit ratio but long-term safety of these drugs should continue to be monitored [32, 33]. This has been identified as a priority for research for the European Union who have commissioned the ADDUCE study (http://adhd-adduce.org), as part of the Seventh Framework Programme for Research and Technological Development, to examine the long-term safety of stimulants.