To investigate suicidality and attention-deficit/hyperactivity disorder (ADHD), this paper aims to systematically review the literature as an extension of previous reviews.

We searched five databases (Ovid MEDLINE, Psychinfo, PubMed, Scopus, Web of Science) with two categories of search terms: (1) suicide; suicidal; suicide behavior; suicide attempt; suicidal thought; and (2) ADHD.

The search resulted 26 articles. There is a positive association between ADHD and suicidality in both sexes and in all age groups. Comorbid disorders mediate between suicidality and ADHD.

Recognizing ADHD, comorbid conditions and suicidality is important in prevention.

Suicide prevention is a public health issue all over the world[1]. Recently, several studies have focused on attention-deficit hyperactivity disorder (ADHD) as a possible psychiatric disorder that may serve as a suicide risk factor as well[2-5]. One of the theoretical backgrounds of it is the construct of impulsivity, which is a well-know personality trait. Impulsivity is a core symptom of ADHD[6,7], moreover it is known, that it correlates to suicidal behavior[8]. Another theoretical background behind the possible association between ADHD and suicide is, that two-thirds of ADHD cases have at least one comorbid psychiatric diagnosis, which most often is conduct disorder, substance use or major depressive episode[9-11]. These comorbid disorders are well-known risk factors of suicide[12-15]. As ADHD is one of the most prevalent (2%-12%) psychiatric disorders among children and adolescents and in 40%-60% of the cases, it continues into adulthood[16,17], all additional knowledge on the possible association between ADHD and suicidality has high clinical importance and can add to suicide prevention.

The growing body of publications on ADHD and suicidality has already resulted in five review or summary papers on this topic. First, James et al[18] searched two electronic databases (MEDLINE, Psychlit) for the period from 1966 to 2003. In their review, they included psychological autopsy studies of teenage and young adult suicides and long-term follow-up studies of ADHD children. They found a positive association between ADHD and completed suicides in males, concluding that ADHD could increase the risk for suicide through comorbid conditions such as conduct disorder and depression. Second, Impey et al[19] performed a search for the period up to January 2011 using three main databases (MEDLINE, EMBASE and PSYCHINFO). They concluded from their review that most suicidal study groups showed a higher rate of ADHD than the controls: At least double the rate for suicidal ideation and around 1.5-2.0 times for suicide attempts and completion. The authors emphasized that comorbidity had a large influence, especially in the cases of delinquency and substance misuse. Third, Furczyk et al[20] published a selective review on the most important currently known associations between ADHD and suicidality. They concluded, similar to the previous reviews[18,19], that there is substantial evidence supporting an association between ADHD and increased suicide risk, and that it is at least partially mediated by comorbidities. They highlight the importance of raising the awareness of health professionals of the risk of suicide in ADHD patients, but further research on the long-term outcomes of the treatment of ADHD patients with a risk of suicide is needed. The selective review paper of Nigg[21] had a wider focus: He overviewed the current knowledge on the health-related impairments of ADHD, including smoking, drug abuse, accidental injury, sleep, obesity, hypertension and diabetes, as well as suicidal behavior. On the topic of ADHD and suicide, the author concluded that ADHD is associated with an elevated risk of suicide attempts (particularly in girls) and completed suicide (particularly in boys), and this risk is mediated by comorbid disorders, which may vary with gender: They include conduct and emotional problems in males and depression in females. Finally, we have to mention Renaud et al[22] summary paper: Based on some selected important research in the field, the authors concluded that there is not a direct link between ADHD and suicide, however, ADHD’s constructs of impulsivity and aggression are related to the development of conduct and oppositional defiant disorders, which can lead to deviancy and drug abuse; all of these comorbid conditions increase the risk of suicide.

Knowing of the growing interest in this topic over the last couple of years, we found it useful to conduct an up-to-date systematic review, which can provide important extensions. The search of James et al[18] was conducted more than one decade ago, and even Impey et al[19] completed their search in January, 2011. The most recently published reviews were not systematic[20-22]. Additionally, all of the reviews have mainly been limited to males[18,19]. Moreover knowing more about the methodology of the studies can lead to a better understanding of the prevalence data. Considering all of this, the current systematic review aims to present an overview on suicidality and ADHD as an extension of the previous ones, not only in the time period of the search, but also by focusing on the following topics: (1) Is ADHD more common in people who are suicidal? (2) Is suicide more prevalent in people with ADHD? and (3) Which other identifiable risk factors can be associated with suicide in ADHD?

Additionally, to be able to compare the included studies, we investigated what kinds of assessments are used for measuring ADHD, suicidality and comorbid conditions.

This review of the last four years strengthens the recent finding that ADHD is related to high suicidality in all age groups and in both girls and boys.

Although our current systematic review was conducted only for the last four years, we still found 26 papers that presented data on ADHD and suicidality. Moreover, we know that several systematic review papers and overviews had been done previously. Impey et al[19], who performed a systematic search on the same topic, covering all studies up to January 2011, the starting point of our search period, found 25 papers. All of them support the view that research on the association of ADHD and suicidality is a subject of high and growing interest, and clinicians and researchers need to have access to up-to-date knowledge in this field.

The studies of this review are culturally diverse, as they come from four continents. This shows that this topic has relevance all over the world and that the conclusions can be used in wider aspects.

Regarding the age groups investigated on the topic of ADHD and suicidality, the studies have been balanced over the last four years: Exactly half of them investigated children/adolescents, and half of them examined adults. This reflects the growing interest in ADHD in adulthood among both healthcare professionals and researchers[81]. Considering the previous studies on this topic - which were included in the review of Impey et al[19] - a majority of them involved the 12-18 age group, although there were a few studies in older and younger age groups as well. Additionally, in their review, Impey et al[19] concluded that, based on the studies included in their review paper, age differences were not clearly definable. In this way, the current review extends our knowledge with further information on all age groups with ADHD and suicidality, including children under 12 and adults, and makes it possible to compare different age groups.

Systematically searching the literature of the last four years, we found only two studies that reported the prevalence data of ADHD among patients with suicidality[24,45]; however, there were a total of six studies addressing that study question in the review of Impey et al[19]. There were five in the review of James et al[18], but four of them were included in the review of Impey et al[19] as well. In both studies in the current review, the diagnoses of ADHD and suicidality were based on a clinician’s opinion. A very important and new result shown in our review is that one-quarter of the suicidal children under 12 years old had ADHD. The prevalence rate of ADHD among suicidal adolescents was lower (5.7%) than in children, and it was lower than in previous studies[19]. One possible explanation could be that these studies did not use either diagnostic interviews or screening tools for the diagnoses of ADHD and suicidality. All of them show that there is still limited data on the prevalence of ADHD among patients with suicidality; however, all of the studies performed found a high prevalence of ADHD in this population, especially among young children. Further studies are needed, but based on the current knowledge, we suggest a routine screening for ADHD patients with suicidal thoughts and attempts, with a special focus on young children.

Almost half of the papers reported prevalence data on suicidality in ADHD patients. The results strengthen the findings of previous studies[18-21] that there is a positive association between ADHD and suicidality, including completed suicides, attempts, as well as ideation. We would like to highlight that, in adolescence, based on the studies of the last four years, more than half of the patients with ADHD had suicidal thoughts, and this prevalence rate is even higher than what has been described previously[19]. It is important to note that even in adulthood, one third of ADHD patients had suicidal ideation. Based on these results, we suggest the introduction of routine screening questions on suicidal thoughts in outpatient/inpatient ADHD clinics, both for those specializing in adults and those specializing in children/adolescents. This improvement in clinical practice can be an important step towards suicide prevention.

The rate of previous suicide attempts was the highest (16%) in the adult ADHD age group; however, adolescents need attention in this respect as well, as almost one-tenth of ADHD patients in this age group had a previous suicide attempt. As one of the strongest predictors of a completed suicide is a previous suicide attempt, close follow-up of these ADHD patients could be of core importance in suicide prevention.

When we examined, which identifiable risk factors can be associated with suicide in ADHD, first we focused on gender. There are still a limited number of studies examining gender differences within this topic. The only study[24] that reported data on the gender distribution of suicidal patients with ADHD is in agreement with the conclusion of previous reviews[18,19] that ADHD is present more often among suicidal men than suicidal women. However, when we examined the prevalence of suicidality (e.g., ideation, attempts and completed suicides) in ADHD patients, two out of three of the studies did not find a difference between men and women[27,37], and one study reported an adjusted estimate for the risk of suicide attempts in females that was almost twice as high[35]. In his selected review paper, Nigg[21] reported that girls had an elevated risk of a suicide attempt as well; however, the author stated that boys have a higher risk of completed suicide among ADHD patients. It is important to note that there are very few studies that have focused on female patients with ADHD[82]. One of them was conducted during the search period of this review[39], while in the previous review on this topic, there were several studies in which only men were enrolled, as well as those in which both females and men were included[19]. Based on the currently available results, both females and males with ADHD need a special focus to recognize their possible suicide risk; however, further studies are needed to gain a better understanding of the gender differences in all age groups.

Second, when we examined identifiable risk factors, which can be associated with suicide in ADHD, we focused on comorbidities. One of the most exciting questions, which also has been raised in all of the previous review and summary papers[18-21], is whether there is a direct association between ADHD and suicidality or if ADHD increases the risk of suicide through comorbid conditions. In the current review, we examined two aspects of this question. First, we reviewed all of the papers within the examined period that measured the prevalence of ADHD in suicidal patients with other psychiatric disorders, such as mood disorders, schizophrenia, alcohol/drug intoxication and chronic fatigue syndrome. The results of all seven studies on this topic showed that the prevalence of suicidality is higher when psychiatric disorders are comorbid with ADHD than in their absence. These findings suggest that the presence of ADHD, as a comorbid condition, conveys an increased risk of suicide for patients with other psychiatric disorders. Second, we investigated the role of comorbidity in ADHD patients with suicidality. The majority of the studies (7/9) found that comorbid disorders mediate between suicidality and ADHD[27,36,38-41,49], which is in line with the conclusion of previous review papers[18-21]. It highlights the importance of raising clinicians’ awareness of the need to screen and treat comorbidity in ADHD, which may reduce suicidality as well.

These findings are limited in that only studies published in English and in Hungarian were included. Three potentially relevant studies were excluded because they were neither in English nor in Hungarian. The vast majority of the studies included in this review have a cross-sectional design, which limits the possible conclusions. Additionally, most of the studies that were conducted before this review paper have a cross-sectional design as well[19]. This should draw the attention of the researchers that, in the future, more studies are needed with a longitudinal design. Additionally, similar to the previous review of Impey et al[19], the measurement methods for both ADHD and suicidality in the studies included in the current review are very different, i.e., diagnostic interviews, rating questionnaires and clinician-made diagnoses - which means that a comparison of the numerical results is not possible.

In conclusion, our systematic highlights that the early recognition and treatment of ADHD - either as a comorbid condition or as a main diagnosis- and the co-occurring psychiatric disorders, can play an important role in the secondary prevention of suicide. Additionally, it could be useful to incorporate routine measurements of suicidality in the daily practice of ADHD clinics.

This work was supported by OTKA K108336 grant. Judit Balázs was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.