Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder which affects 5%-7% of youths worldwide [1,2]. Symptoms and associated functional impairments are now understood to continue throughout adulthood in approximately 90% of cases [3]. Adults with ADHD is associated with devastating behavioral and neuropsychiatric outcomes including social impairment, accidents and unintentional injuries, risky sexual behaviors, substance abuse, educational and occupational impairments [4,5].

Pharmacological treatment with ADHD medications significantly improves ADHD symptoms, functional impairment, psychiatric comorbidities and health-related quality of life among adults with ADHD [6]. However, there are concerns regarding the safety of ADHD medications including the potential for misuse, abuse and dependence of ADHD medications [7]. Many concerns that adults may misuse ADHD medication for performance enhancement or recreational purposes, leading to abuse and dependence. A previous review suggested that misuse and diversion of prescription stimulants is found in 5%-10% of high school students and 5%-35% of college students, while non-stimulant medications appear to have no abuse potential [7].

In this regard, the Korean Ministry of Food and Drug Safety has started to monitor methylphenidate prescriptions for drug abuse prevention measures. Prescription records of methylphenidate are reported to the Narcotics Information Management System to be analyzed. Clinicians who are suspected of excessive prescriptions of methylphenidate are subject to intensive inspections. These policies, though well-intentioned, may inadvertently discourage clinicians from prescribing ADHD medication in adults, especially those who were not diagnosed of ADHD in their childhood.

Nationwide rate of ADHD pharmacotherapy in the Korean adult population increased from 0.01% to 0.03%, which is very low compared with the global average [8]. Moreover, recent studies indicate that among adults with ADHD, low medication adherence and long-term treatment persistence remain major challenges. A previous study reported that early medication discontinuation is prevalent in ADHD treatment, particularly among young adults [9]. Only 48% of adults remained on treatment within 1 year of initiation. Another study reported that of those treated with ADHD medication, less than a third adhered to medication [10]. Moreover, nonadherence in ADHD was associated with a range of adverse outcomes, which supports the argument that adherence matters [11]. However, research on medication adherence in adults ADHD remains scarce in Asian populations, especially within a Korean clinical context.

Therefore, the present study aimed to examine the clinical characteristics of adults newly diagnosed with ADHD in their adulthood and to evaluate their medication adherence, persistence, and substance-related events. We sought to clarify that the major clinical challenge in treating adult ADHD lies not in the risk of medication abuse but in sustaining adequate adherence to pharmacological treatment.

The eligible sample consisted of 174 adults with ADHD (mean age, 25.14±7.84; male, 73.6%; female, 26.4%). The sociodemographic and clinical characteristics of the subjects are presented in Table 1.

ADHD core symptoms (56.3%) such as inattention, hyperactivity and impulsivity were the most common chief complaints at first visit of the subjects (Table 2). Besides ADHD core symptoms, interpersonal relationship problems (41.4%) were the most common chief complaint. Depressed mood (34.5%), anxiety (33.3%), irritability (28.7%), insomnia (27.6%) were also common chief complaints of the subjects.

One or more psychiatric comorbidities were observed in 76.4% of the subjects: 47.1% had 1, 23.6% had 2, and 5.2% had 3 or more comorbidities (Table 3). The most common comorbid diagnosis of the subjects was depressive disorder (37.9%). Anxiety disorder (28.2%) and insomnia disorder (14.4%) were also common.

Of the 174 subjects, 149 (85.63%) started pharmacological therapy after the ADHD diagnosis: 94 (63.09%) were treated with methylphenidate and 55 (36.91%) were treated with atomoxetine. The average of the therapeutic dose of methylphenidate prescribed in the methylphenidate group was 46.03±16.53 mg. The average of the therapeutic dose of atomoxetine prescribed in the atomoxetine group was 64.67±20.01 mg. None of the subjects developed any substance-related events regarding ADHD medication during the 2-year period.

Adherence was 65.2% in the methylphenidate group and 67.9% in the atomoxetine group (Table 4). 42.6% of methylphenidate group showed higher adherence than 80%, and 40.0% of atomoxetine group showed higher adherence than 80%. There was no significant difference between the two groups in regard to adherence. Persistence was 188.57±210.95 days in the methylphenidate group and 214.91±232.98 days in the atomoxetine group. Only 13.8% of methylphenidate group and 18.2% of atomoxetine group showed longer persistence than 365 days. There was no significant difference between the two groups in regard to persistence.

In this study, only 56.3% of the adults with ADHD reported ADHD symptoms as their chief complaints when they visited the psychiatric clinic. Almost half of the adults with ADHD sought treatment because of other psychiatric symptoms such as interpersonal relationship problems and depressed mood. This aligns with previous research indicating that it is often the resultant functional impairments, rather than the mere presence of ADHD symptoms, that drive individuals to seek treatment [13]. This finding suggests that, in adults, ADHD symptoms are often masked by or expressed through secondary emotional or relational problems, leading to delayed or missed diagnosis. Previous international studies have similarly shown that adults with ADHD frequently present with mood or anxiety complaints rather than hyperactivity or inattention, contributing to underrecognition and undertreatment of the disorder [14]. Therefore, when evaluating adults who present with depression, anxiety, or interpersonal problems, clinicians should routinely assess for underlying ADHD, as appropriate diagnosis and treatment of ADHD can substantially improve both core symptoms and associated functional or emotional difficulties. Research provides evidence that pharmacological treatment can be effective not only in treating ADHD symptoms, but also alleviating its functional consequences as well [6]. Studies showed that methylphenidate and atomoxetine both improved social functioning as well as comorbid depression of adults with ADHD [15,16].

The results of this study also showed that 76.4% of adults with ADHD had one or more psychiatric comorbidities. Adults who are diagnosed with ADHD in their adulthood are those who did not get any intervention for their ADHD during their childhood. Consequently, they may be more susceptible to developing psychiatric comorbidities and experiencing greater functional impairments. A large cross-sectional study found adults with ADHD who had received treatment in childhood were significantly less likely to be out of work compared to those never having received treatment [17]. Furthermore, a systematic review showed that ADHD medications were associated with a reduced risk of developing depression in children and adolescents with ADHD [18]. These findings suggest that adults diagnosed with ADHD in their adulthood may have a heightened need for adequate treatment since they experienced prolonged periods without appropriate recognition or intervention of their ADHD.

Findings from this study indicate that adherence for methylphenidate and atomoxetine were 65.2% and 67.9% respectively, which is low. Those who continued their ADHD pharmacological treatment for more than 1 year was only 13.8% and 18.2% in methylphenidate and atomoxetine group, respectively. This is in line with previous studies that showed mean adherence rate ranging from 52% to 87% [19]. On a study on pharmacy database of adults with ADHD, 80% of patients had stopped pharmacological treatment at the end of 15-month observation period [20]. Unlike children with ADHD who are reliant on their parents, adults living independently may be more vulnerable to non-adherence to treatment. There are studies that report adults with ADHD have a slightly lower therapeutic response rate (-60%) compared to those of children with ADHD [21]. Low adherence to medication has been suggested as one of the potential causes of clinically significant residual symptoms despite medication treatment in adults with ADHD [22]. In a previous study, adults with ADHD with less than 80% adherence had substantially more residual symptoms of ADHD compared to those whose adherence was higher [20].

The effectiveness and speed of action of stimulants have been suggested for the potential for abuse and misuse [7]. To the contrary, data suggests a lack of abuse potential and lack of actual medication misuse for the nonstimulant medications such as atomoxetine [23]. In this study, there was no significant difference between the methylphenidate and atomoxetine regarding adherence and persistence. Moreover, none of the subjects had any substance-related events regarding ADHD medication throughout the 2-year period. Hence it could be inferred that misuse and abuse of methylphenidate may not be common for those who receives proper diagnosis and adequate prescription for ADHD. A more pressing concern in clinical practice for adults with ADHD is their low treatment adherence, which can significantly contribute to suboptimal therapeutic outcomes.

There are some limitations of this study that should be addressed. First, the study was based on a retrospective chart review. Second, the subjects represent a specific clinical population, and the generalizability of the results may be limited. Third, factors associated with low adherence and persistence of the participants, as well as their therapeutic response were not examined. It is unclear how these factors may have influenced the current findings. Previous studies have suggested that age, education level, comorbid psychiatric conditions, treatment perceptions, social or family support may influence adherence to ADHD medication in adults [24]. Future studies should explore the individual factors contributing to poor adherence and investigate strategies to support sustained engagement in treatment among adults with ADHD.

In conclusion, ADHD not only persists into adulthood but is associated with a wide range of psychiatric symptoms and comorbidities. Given that ADHD is the most common neurodevelopmental disorder, and remains significantly underdiagnosed particularly in adults, clinicians should constantly consider the possibility of ADHD when evaluating adult patients. Most importantly, because the risk of ADHD medication abuse is minimal for those who receive proper ADHD diagnosis and management, greater emphasis should be placed on improving adherence to pharmacological treatment. Enhancing treatment adherence in adults with ADHD has the potential to significantly improve functioning across multiple domains, ultimately benefiting not only affected individuals but also the broader society through improved productivity and reduced healthcare burden.