Our study is the first we know of to assess predictors of medication whist adjusting for ADHD symptom severity. Girls with ADHD were less likely to be prescribed medication even when they display similar ADHD symptom levels to boys. Conduct problems also predicted medication independently of ADHD symptoms, at both age seven and 14. Our findings show that these factors are important predictors of ADHD medication, even when symptom severity is accounted for.

Existing studies have reported that 36–43% of children with ADHD in the UK are prescribed medication [1, 27, 28]: lower than rates reported in Europe that range from 43 to 67% [7, 10, 29]. Recent analysis conducted in the USA with similar methodology to the study we report here found that 62% of 2–17 year olds with current ADHD were taking medication in 2016 [5]. Rates of medication use in Canada have recently been estimated at 70% for those aged up to 24 [6], and in East Asia 62% of young people with a new diagnosis of ADHD initiated pharmacological treatment in a 1-year observational study [9]. Existing studies that previous UK estimates derive from use small samples, primary care data or are over 10 years old: as such there is a need for further research in this area, especially given concern in the scientific literature and media about prescription rates for ADHD [30]. In the current study, 45.6% of children with ADHD were reported to be taking medication at age 14. The proportion of children with ADHD using medication remains lower than in North America, East Asia, France and Central Europe and in line with recent statistics from Germany (43.1% in 14 year olds in 2014) [5,6,7, 9, 10, 29].

As our data include only those who were taking medication at age 14 this may be an underestimate of the true prevalence of medicated ADHD as we would not capture those who discontinued prior to age 14. Having said this, prevalence of medication use for childhood ADHD has been reported to peak at age 13–14 [29]: it will be of interest to explore whether the proportion of children in the MCS using medication subsequently decreases.

There were three peaks in age of medication initiation: at ages seven, nine and 12. At age seven, children in UK schools are subject to the first national tests. Consequently, there are increasing demands on cognition, behaviour and attention with children expected to remain seated and focussed [31]. The peak at age 12 corresponds to recently entering secondary school. The social and organisational demands of secondary schools are a large ‘jump’ from primary schools: children are expected to organise and bring their own study materials, and move from class to class throughout the day. It is likely that children with ADHD who could cope with demands at primary school level, where staff and location are consistent, may struggle more than their peers upon entry to secondary school [32] and medication may, therefore, become a viable option to enable children to function at their best: indeed one of the major long term impacts of ADHD is poor educational attainment [33]. In addition at age 12 children are entering adolescence, and physical, emotional and mental changes associated with this may be linked to the initiation of medication.

We found medication was initiated at age three and four in a small number of children (4% of those who received medication). A larger proportion received medication at age five. Previous UK reports have been limited to children age six and above [34]. Early medication of children has been a source of concern for ethicists [35] who have argued that very young children typically show inattention and hyperactivity [36], thus it may not be appropriate. Alternately, it may be that children who are prescribed medication at such a young age comprise a group with severe symptoms that has led to family stress or exclusion from child care. Further research should explore this.

Many studies have found that more severe ADHD is associated with medication use [7, 9, 10, 27]. Gender also predicts more severe symptoms of ADHD [30]. We found that symptom severity was a powerful predictor of ADHD medication, with children who took medication suffering from more severe symptomology than those who did not. This finding aligns with clinical guidelines that suggest non-pharmacological treatment should be the first line of treatment for ADHD [37], so we would expect medication to be prescribed only for more severe cases.

Taking symptom severity into account, boys were over three times more likely to be taking medication for their ADHD than girls: a finding not seen in some other studies [5,6,7]. Treatment choice and decisions may be influenced by gender bias in beliefs about how likely the child is to benefit from the treatment, and stigma operating differentially by child gender [4]. It could be that girls are perceived by parents, teachers or clinicians as less likely to benefit from pharmacological treatment or more likely to benefit from behavioural treatments: evidence in this area is lacking. Our study provides evidence that even if recognised, girls may face additional barriers to access treatment: clinicians should be aware of this. Alternatively, some have argued ADHD is too frequently diagnosed in boys, and that clinicians think a prototypical ADHD child is male. This might lead to boys being more often recognised and treated [38].

The second significant predictor of whether children received ADHD medication was conduct problems. The SDQ-CP subscale asks about temper, obedience, fighting, lying and stealing. Our finding is in line with other studies that report co-occurring conduct symptoms to be more frequent in those who receive medication for ADHD [5, 7, 8, 10]. This raises debate as to whether medication for ADHD is being prescribed to those who display problematic behaviour beyond ADHD, or whether those with the most severe or complex behavioural difficulties are taking medication because of this complexity. A moderator analysis of the multimodal treatment for ADHD study found that children with comorbid conduct or oppositional defiant disorder at baseline responded similarly to treatment to those without these comorbidities [39]: medication was not disproportionately beneficial to these children. However, a recent systematic review reports evidence that both psychostimulants and atomoxetine are effective in reducing disruptive and aggressive behaviours in children with ADHD [40]. Our findings raise questions about diagnostic silos and whether children with both ADHD and conduct disorder should be viewed through the ADHD or conduct disorder lens. If medications currently licensed to treat ADHD are beneficial for a primary diagnosis of conduct disorder, consideration of whether UK clinical guidelines and licensing should be changed is needed. The National Institute for Health and Care Excellence (NICE) ADHD guideline recommends non-pharmacological treatments as the first option for those with ADHD and symptoms of conduct disorder [37], however, guidelines from Canada and other countries recommend medication in these cases (e.g., [41, 42]). Clinicians should be aware that severity of problems in comorbid ADHD and conduct disorder may be driven by the latter. Monitoring treatment response is, therefore, essential to ensure medication is beneficial in these cases.

Although children with ADHD were of lower SES than the MCS cohort, no measure of SES was associated with ADHD medication use. This is encouraging as it suggests that the current system is free from bias in either direction: it has been argued that low SES may preclude individuals from medication access because of the affordability of medication, or stigma around the perceived cause of the child’s symptoms. As the UK healthcare system is free at the point of use, and prescriptions are free for those on low incomes, our results may reflect societal organisational structures that allow healthcare access equally to those from different socioeconomic backgrounds.

Previous studies [11, 43], found an association between medication and socioeconomic disadvantage, but reported opposing directions of effect. The study by Froehlich et al. using national survey data in the USA found low SES to be a barrier to medication use, whereas the Swedish register-based study of Hjern et al. finds socioeconomic disadvantage to be a strong predictor of ADHD medication use. There appears to be an interaction between ADHD being more prevalent and severe in those who grow up in socioeconomically disadvantaged households [44] and the healthcare system in the country where the study is conducted. Our findings highlight that each healthcare system will have its own biases in prescribing practices and throw the status of low SES as a barrier to medication into question.