Background Attention deficit hyperactivity disorder (ADHD) is a common disorder that is associated with criminal behavior. Pharmacological treatment is available for ADHD and may reduce the risk of criminality Methods We gathered information on all individuals with a diagnosis of ADHD (N=25,656), their pharmacological treatment, and subsequent criminal convictions in Sweden during 2006 to 2009 using Swedish national registers. We used stratified Cox regression analyses to compare the rate of criminality while on ADHD medication, compared with the rate for the same individual while off medication. Results Compared to non-medication periods, the criminality rate while on medication was significantly decreased by 32% (stratified Cox Regression hazard ratio: 0.68; 95 % confidence interval: 0.63-0.73) for men and 41% (hazard ratio: 0.59; 95 % confidence interval: 0.50-0.70) for women. The rate reduction remained between 17-46% in sensitivity analyses among males, including different exposures (e.g., type of treatment – stimulant and non-stimulant) and outcomes (e.g., type of crime - less severe, violent, and substance-related conviction). Conclusions We found statistically significant associations between ADHD medication and criminality in within-individual comparisons, with lower rates of criminality observed during periods on treatment. These findings raise the possibility that medication treatment reduces the risk of criminality among patients with ADHD.

Twins, full siblings, and half-siblings are increasingly used as comparison groups in matched cohort and matched case-control studies. The "within-pair" estimates acquired through these comparisons are free from confounding from all factors that are shared by the siblings. This has made sibling comparisons popular in studying associations thought likely to suffer confounding from socioeconomic or genetic factors. Despite the wide application of these designs in epidemiology, they have received little scrutiny from a statistical or methodological standpoint. In this paper we show, analytically and through a series of simulations, that the standard interpretation of the models is subject to several limitations that are rarely acknowledged.Although within-pair estimates will not be confounded by factors shared by the siblings, such estimates are more severely biased by non-shared confounders than the unpaired estimate. If siblings are less similar with regard to confounders than to the exposure under study, the within-pair estimate will always be more biased than the ordinary unpaired estimate. Attenuation of associations due to random measurement error in exposure will also be higher in the within-pair estimate, leading within-pair associations to be weaker than corresponding unpaired associations, even in the absence of confounding. Implications for the interpretation of sibling comparison results are discussed.

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Objective To investigate the association between drug treatment for attention-deficit/hyperactivity disorder (ADHD) and risk of concomitant suicidal behaviour among patients with ADHD.Design Register based longitudinal study using within patient design.Setting Linkage of multiple national registers in Sweden.Participants 37 936 patients with ADHD born between 1960 and 1996 and followed from 2006 to 2009 for treatment status by ADHD drug treatment and suicide related events (suicide attempt and completed suicide).Main outcome measure Incidence rate of suicide related events during ADHD drug treatment periods compared with that during non-treatment periods.Results Among 37 936 patients with ADHD, 7019 suicide related events occurred during 150 721 person years of follow-up. At the population level, drug treatment of ADHD was associated with an increased rate of suicide related events (hazard ratio 1.31, 95% confidence interval 1.19 to 1.44). However, the within patient comparison showed a reverse association between ADHD drug treatment and rate of suicide related events (0.89, 0.79 to 1.00). Among stimulant users, a reduced within patient rate of suicide related events was seen during treatment periods (0.81, 0.70 to 0.94). Among non-stimulant/mixed users, no significantly increased within patient rate of suicide related events during non-stimulant treatment periods was seen (0.96, 0.72 to 1.30).Conclusions This study found no evidence for a positive association between the use of drug treatments for ADHD and the risk of concomitant suicidal behaviour among patients with ADHD. If anything, the results pointed to a potential protective effect of drugs for ADHD on suicidal behaviour, particularly for stimulant drugs. The study highlights the importance of using within patient designs to control for confounding in future pharmacoepidemiological studies.