The study results published by Mohammad Effatpanah and colleagues  provided some food for thought recently on the topic of whether there may be an *association* between "serum magnesium levels and the diagnosis of attention deficit hyperactivity disorder (ADHD)."
The name of the research game was meta-analysis, that well used 'boiling down' of the published (hopefully peer-reviewed) science literature into something like a coherent 'conclusion'. The starting point for Effatpanah was that: "Current research suggests conflicting evidence surrounding the association between serum magnesium levels and the diagnosis of attention deficit hyperactivity disorder (ADHD)." It's interesting that this isn't the first time that magnesium and ADHD has been put under the meta-analysis microscope  and that particular meta-analysis didn't suggest such conflict.
Never mind. Seven studies made the grade for Effatpanah, together revealing that "subjects with ADHD had 0.105 mmol/l (95% CI: -0.188, -0.022; P < 0.013) lower serum magnesium levels compared with to their healthy controls." Researchers did also talk about 'high heterogeneity' across the studies analysed. This indicates that whilst there may well be "an inverse relationship between serum magnesium deficiency and ADHD" overall, the individual studies included in their meta-analysis weren't always in agreement with one and another.
So what conclusions can we take from the Effatpanah and other (meta-analysis) studies in this area? Well, more investigation is required on the suggestion of a *link* between magnesium and ADHD. We need to know more about the biology of why reduced biological levels of magnesium might be important to ADHD or ADHD-type behaviours  and whether something as simple as supplementing with magnesium *might* make a difference for some people  (minus any medical or clinical advice from me on this or any other topic). Indeed, on that last issue, I might refer you back to some other occasions where magnesium has been mentioned in the context of nutritional intervention for ADHD (see here). I'm also inclined to mention that there may be other 'labels' where magnesium might require a little more study (see here), some of which might 'overlap' with a diagnosis of ADHD. And of course, we should remember that magnesium 'issues' in the context of autism might not be the end of the story when it comes to trace metals and ADHD (see here)...
 Effatpanah M. et al. Magnesium status and attention deficit hyperactivity disorder (ADHD): A meta-analysis. Psychiatry Res. 2019 Feb 19;274:228-234.
 Huang YH. et al. Significantly lower serum and hair magnesium levels in children with attention deficit hyperactivity disorder than controls: A systematic review and meta-analysis. Prog Neuropsychopharmacol Biol Psychiatry. 2019 Mar 2;90:134-141.
 Black LJ. et al. Low dietary intake of magnesium is associated with increased externalising behaviours in adolescents. Public Health Nutr. 2015 Jul;18(10):1824-30.
 Ghanizadeh A. A systematic review of magnesium therapy for treating attention deficit hyperactivity disorder. Arch Iran Med. 2013 Jul;16(7):412-7.