We know that ADHD is linked to substance use, but as is often the case with psychology research, there are some challenges in drawing conclusions about cause-and-effect.
If ADHD symptoms put people at risk for substance use, that has different implications than if substance abuse puts people at risk for more ADHD symptoms. But if you do a study and find that people with higher ADHD symptoms have more substance use, that unfortunately doesn’t tell you which direction the cause-and-effect goes!
You might intuitively suspect that ADHD symptoms are more likely to cause substance use than the other way around, and a new study adds some evidence for that view.
The authors of the study used a type of experiment that uses known genetic associations to shed light on cause-and-effect between different conditions. In this case, they looked at genetic variants thought to be specifically associated with ADHD and genetic variants thought to be specifically associated with substance use. If the ADHD variants increase substance use risk, that would suggest that ADHD to some extent causes substance use, and vice-versa if the substance use variants increase ADHD risk.
If you want all the gory scientific details about this type of study, here is an article that gets into it. But the important thing to know is that while this kind of study isn’t foolproof, it can give some useful information about how different conditions are linked causally.
Looking at known genetic variants in this way, the researchers found that genetic variants thought to be specifically associated with ADHD had all of the following effects:
- Increasing people’s likelihood of taking up smoking
- Increasing smokers’ tendency to smoke heavily
- Decreasing smokers’ likelihood of quitting smoking
- Increasing people’s likelihood of starting cannabis use
The study also turned up some weak, inconclusive evidence of a link between genetic variants associated with ADHD and alcohol dependence.
By contrast, there wasn’t much convincing evidence of any causality going in the opposite direction – that is, of genetic variants associated specifically with substance use increasing people’s ADHD risk. On the whole, those findings are much more consistent with the idea that ADHD causes substance use than the reverse.
For ADHDers, that result makes a lot of sense. We know that impulsivity, reward-seeking behavior and a need to self-medicate are all factors that can tip people with ADHD over into a drug, alcohol or smoking habit.
For mental health professionals and public health officials, meanwhile, these findings underscore the fact that ADHD doesn’t just correlate with substance use but may play a causal role – which means that if we want to reduce substance use, we also need to prioritize ADHD treatment!
Image: Flickr/Matthias Ripp