Nationwide cohort study: methylphenidate reduces traffic accidents in persons with ADHD

Taiwan has a single-payer healthcare system that covers virtually every inhabitant (99.5%). That makes it relatively easy to track healthcare issues using its comprehensive National Health Insurance Research Database.

This database maintains a subset, the Longitudinal Health Insurance Database (LHID), consisting of a million persons, with no significant differences in sex, age, or healthcare use from the parent database.

A Taiwanese research team used the LHID to identify 114,486 individuals diagnosed with ADHD from 1997 to 2013. It then compared their motor vehicle (including motorcycles, which are extremely common in Taiwan) crash patterns with 338,261 normally developing controls from the same database.

Adjusting for sex, age, and psychiatric comorbidities, persons with ADHD were about a fifth (19%)more likely to be in traffic crashes. Breaking it down further by sex, women with ADHD were no more likely to be in crashes, but men with ADHD were about a quarter (24%) more likely than their healthy counterparts.

Since the database also tracks pharmaceutical prescriptions, the team also looked into the effect of methylphenidate (MPH), the medication that is the first-line treatment for ADHD under Taiwanese guidelines, and the only approved stimulant. Atomoxetine, a non-stimulant, is used where MPH is either ineffective or not indicated for any other reason and is only used in 4% of all cases.

Of the 114,486 persons diagnosed with ADHD, 89,826 used MPH, and 24,660 did not.

Compared with persons with ADHD who were not on methylphenidate, those with ADHD who were on MPH for 180 days (roughly half a year) or less had 77% fewer accidents, and those on MPH for over 180 days had 93% fewer accidents. This strong dose-response relationship is suggestive of a causal relationship, with MPH perhaps reducing impulsive behavior, particularly among young men with ADHD.

The team also conducted within-person analyses, comparing times when persons with ADHD were taking MPH with periods when they were not. These showed no effect within 30 days of use, rising to a 65%reduction in crashes within 60 to 90 days of use, which was barely outside the 95% confidence interval (p = .07), very likely because of “the extremely low incidence of transport accidence (i.e. 0.6%)enlarged the confidence interval.”

The authors concluded, “All registration medical claim data came from the nationally-representative sample of NHI, minimizing the selection and recall bias. By excluding transport accidents before ADHD diagnosis, we have precluded the reverse association between ADHD and road traffic accidents as much as possible. The advantage of the between-subjects comparison was that we were able to examine the MPH effect in different dose groups. However, confounding by indication cannot be eliminated. For example, those with a severe degree of ADHD symptoms, an exhibition of risky behaviors, or comorbid with other psychiatric illnesses were more likely to be prescribed medication. Hence, we also performed within-subject comparisons to adjust for time-invariant factors.”

Transport safety thus offers another compelling reason to treat ADHD symptoms. Methylphenidate in particular seems to be especially effective in reducing traffic fatalities and injuries.

Yi-Chun Liu, Vincent Chin-Hung Chen, Yao-Hsu Yang, Yi-LungChe, and Michael Gossop, “Association of psychiatric comorbidities with the risk of transport accidents in ADHD and MPH,” Epidemiology and PsychiatricSciences30 (2021), e14, 1-9,