Iron, Zinc, Omega-3 and Magnesium in ADHD: What the Evidence Actually Ranks

Your ADHD app tracks your tasks. It doesn't track whether your brain has the chemical building blocks to do them.

You've got your notifications dialled in. Your to-do list is colour-coded. You've subscribed to three different productivity apps. But you skipped breakfast because you forgot to eat, and you're running on your fourth coffee because focus feels impossible today.

Here's the thing nobody in the productivity space wants to tell you: your brain might not have the nutrients it needs to synthesise dopamine in the first place. And no app can fix that with a timer.

The ADHD neurobiology is real. Dopamine dysregulation is real. But so is iron deficiency. So is zinc deficiency. And a body of research links micronutrient status to ADHD symptom severity — iron most consistently of all (Konofal et al., 2004). That's not "nutrition cures ADHD." It isn't even "nutrition treats ADHD." It's narrower and more useful than that: your brain builds its chemistry out of raw materials, and whether you're short of them is one of the few things on that list you can actually go and check.

This is the conversation the app industry won't have. Because no SaaS company profits from you eating sardines.

Your ADHD Might Feel Worse Because of What's on Your Plate

Let's separate fact from fiction. ADHD is genetic. The underlying dopamine dysregulation isn't fixed by eating better. But ADHD symptom severity is not purely genetic—it's influenced by the raw materials your brain has to work with.

Think of it this way: the enzymes that build dopamine and noradrenaline need specific raw materials to work at all — tyrosine hydroxylase, the rate-limiting one, has an iron atom in its active site (Daubner et al., 2011). Your genetics set how that system runs. Nutritional status doesn't change the genetics, but it can decide whether the machinery has what it physically needs.

The evidence here is uneven, though, and it's worth knowing which parts are solid. Konofal et al. (2004) in the Archives of Pediatrics & Adolescent Medicine found that children with ADHD had substantially lower serum ferritin (iron storage) than controls — a well-replicated association. Mousain-Bosc et al. (2006) in Magnesium Research gave magnesium with vitamin B6 to 40 children with ADHD and reported improvement, but that study was open-label: no randomisation, no placebo arm, no blinding. Differences in iron, magnesium and zinc status between ADHD and control groups turn up repeatedly across the literature. What varies enormously is whether correcting them has been shown to help.

One thing to be precise about, since supplement marketing routinely blurs it. Under EU and Great Britain nutrition and health claims law, the European Food Safety Authority (EFSA) has authorised claims for these nutrients — iron and zinc contribute to normal cognitive function, magnesium to normal functioning of the nervous system, DHA to maintenance of normal brain function (EU Register on nutrition and health claims). Those are claims about maintaining normal function in the general population. None is a claim about treating, preventing or improving ADHD.

And here's the honest part: where nutrient effects have been measured at all, they are modest. Nothing here comes close to what's reported for stimulant medication, and no trial produces a percentage you can apply to yourself. Treat any specific number attached to a supplement as marketing.

And none of this is a replacement for medication. This isn't "eat magnesium instead of taking Ritalin." It's "your Ritalin might work better if your brain isn't simultaneously starving for micronutrients."

The Big Four: What Your Brain Is Actually Missing

Four nutrients come up again and again in ADHD conversations: iron, zinc, omega-3 and magnesium. They do not have equal evidence behind them, so they're ordered here by how much is actually known rather than by how often they're recommended. Magnesium is last for a reason.

Iron — The Myelin Architect

Iron is essential for two things: myelination (the insulation that speeds up neural signalling) and catecholamine synthesis. In tyrosine hydroxylase — the rate-limiting enzyme for building dopamine and noradrenaline — the iron "must be in the ferrous state to carry out catalysis" (Daubner et al., 2011). That is a specific, named, physical role, and it's the clearest mechanism any nutrient has in this whole area.

A mechanism is not a diagnosis, though. Iron being needed to build dopamine does not mean low iron causes ADHD, or that topping it up will fix anything.

Konofal et al. (2004) measured serum ferritin in 53 children with ADHD against 27 controls. Mean ferritin was 23 ng/mL (±13) in the ADHD group versus 44 ng/mL (±22) in controls (P<.001) — and 84% of the ADHD children had ferritin below 30 ng/mL, compared with 18% of the controls. Below 30 ng/mL is the threshold that research uses for depleted stores. Note what this study is, though: cross-sectional. It shows low ferritin and ADHD turning up together. It involved no supplementation at all.

What it looks like when you're deficient:

• Chronic fatigue that coffee doesn't touch
• Weak working memory and poor focus
• Emotional dysregulation (irritability, mood swings)
• Restless legs at night
• Brittle hair/nails, pale skin

What the research says: Konofal et al. (2008) ran a small placebo-controlled pilot: 23 non-anaemic children whose ferritin was already below 30 ng/mL, over 12 weeks. ADHD Rating Scale scores fell on iron (−11.0, ±13.9) but not on placebo (+3.0, ±5.7). Read the entry criteria before the headline. Every child enrolled was already iron-depleted, and the groups were lopsided — 18 on iron, 5 on placebo. So the accurate statement is not "iron works only in deficient children and does nothing in everyone else." It's that iron supplementation is untested in children whose iron stores are normal. Untested is a different thing from shown not to work.

The honest caveat: Iron is the one nutrient here that can seriously hurt someone. Acute iron overdose has historically been a leading cause of fatal poisoning in young children, and the toxic threshold depends on body weight. Get tested first — serum ferritin — and keep supplements out of reach of children. We wrote this one up on its own because it needs the room: iron, ferritin and the blood test to ask for.

Zinc — The Executive Function Co-Star

Zinc is a cofactor for hundreds of enzymes and is involved in the modulation of dopamine, GABA and glutamate signalling — the systems underneath reward processing and impulse control.

Bilici et al. (2004) in Progress in Neuro-Psychopharmacology and Biological Psychiatry randomised 400 children with ADHD to zinc sulfate or placebo for 12 weeks. Zinc beat placebo on hyperactivity, impulsivity and impaired socialisation — but not on attention, which is the symptom most readers of this piece care about most. The therapeutic response rate was 28.7% on zinc versus 20% on placebo. The trial also had a high dropout rate and no intention-to-treat analysis.

What it looks like when you're deficient:

• Weak immunity (frequent infections)
• Poor wound healing
• Severe executive dysfunction (especially task initiation)
• Emotional dysregulation
• Hair loss

What the research says: Both of the positive clinical trials of zinc in ADHD come from Turkey and Iran — regions with suspected endemic zinc deficiency (Arnold & DiSilvestro, 2005). That is the cleanest illustration in this whole field of an effect that is about correcting a deficiency rather than treating ADHD, and it's the reason these results may not carry over to a well-fed population.

The honest caveat: Zinc has a narrow window. More is not better. Above 40mg daily, you risk copper deficiency (which causes neurological problems). NHS guidance is more conservative still — it advises against taking more than 25mg a day in supplement form unless a doctor recommends it, so UK readers should treat that as the ceiling and check with their GP or pharmacist first. Supplementing without knowing your baseline is guessing.

Omega-3 — The Membrane Integrity Guardian

Omega-3 fatty acids (EPA and DHA) are structural components of neuronal membranes and are involved in dopamine receptor signalling. DHA is the most abundant omega-3 fatty acid in the brain.

ADHD brains often have lower omega-3 status. Whether that's cause or consequence is still debated. But low omega-3 correlates with worse symptoms.

What the research says: Bloch & Qawasmi (2011) published a meta-analysis in the Journal of the American Academy of Child & Adolescent Psychiatry covering 10 randomised controlled trials of omega-3 supplementation in ADHD and 699 participants. The pooled effect was modest — around 0.3 in effect-size terms. They also found that the EPA dose within supplements correlated with efficacy. Note that carefully: EPA dose, not the EPA-to-DHA ratio.

That finding has since been overtaken. Liu et al. (2023) in the Journal of Clinical Psychiatry pooled 22 randomised trials and 1,789 participants, found no significant effect on core ADHD symptoms overall (SMD −0.16, 95% CI −0.34 to 0.01), and reported explicitly that neither a high EPA dose nor a high EPA:DHA ratio improved symptoms. Only the subgroup of studies running four months or longer reached significance. So the "get the EPA:DHA ratio right" rule you'll see everywhere isn't settled science — it's a 2011 signal that a much larger 2023 evidence base did not reproduce.

Worth correcting one thing that circulates constantly: flax is not a DHA source. Flax provides ALA and contains no DHA at all. The body converts ALA to EPA at roughly 5–10%, and to DHA far less efficiently — frequently under 1%.

NICE guideline NG87 also declines to recommend dietary fatty acid supplementation for treating ADHD in children and young people, advising instead a balanced diet including two weekly portions of fish, one of them oily (NICE NG87).

What it looks like when you're deficient:

• Dry, flaky skin
• Mood instability and depression
• Poor memory and brain fog
• Weak immune function
• Joint and muscle aches

The honest caveat: The best estimate of the effect is small, and the newest and largest estimate is null. Omega-3 is better thought of as general maintenance than as a symptom reducer. Most people on Western diets are low in it anyway, so this is universal value, not ADHD-specific.

Magnesium — The Popular One With the Thinnest Evidence

Magnesium is the ADHD supplement — the one in every thread, every comment section, every "what I take for my brain" list. It also has the weakest evidence of anything in this piece: there is no randomised, placebo-controlled trial showing that magnesium supplementation beats placebo for ADHD symptoms.

What exists instead is biomarker work and open trials. Kozielec and Starobrat-Hermelin (1997) examined 116 children aged 9–12 and found magnesium deficiency on at least one of three measures in 95% of them. Read the breakdown before quoting the headline: hair 77.6%, red blood cells 58.6%, serum 33.6%. Hair mineral analysis is not a validated clinical measure of magnesium status, and serum magnesium is a poor proxy for total body magnesium. The figure is real as published. Quoted bare, it misleads.

What it looks like when you're deficient:

• Muscle twitching or tremors
• Poor sleep quality (especially early morning waking)
• Anxiety and racing thoughts
• Brain fog and slow processing
• Weak stress tolerance

What the research says: Mousain-Bosc et al. (2006) gave magnesium with vitamin B6 to 40 children with ADHD for at least eight weeks. Symptoms improved, red blood cell magnesium rose, and symptoms returned when the supplement stopped. It is widely described online as a randomised controlled trial. It wasn't — it was open-label, with no randomisation, no placebo arm and no blinding, and its 36-child comparison group existed for baseline biomarker measurement only. Without a placebo group, "people improved on magnesium" and "people improved" are the same sentence.

The honest caveat: "No good evidence" is not the same as "shown not to work." Magnesium hasn't failed a fair trial for ADHD; it hasn't had one. It may well help some people — nobody knows. The longer version is here: magnesium for ADHD, and why the argument about forms misses the point. One safety note on that 2006 combination, since supplement pages rarely carry it: EFSA's 2023 opinion set the tolerable upper intake level for vitamin B6 at 12mg a day for adults, because higher intakes are linked to peripheral neuropathy. Combination products stack.

Beyond the Four: What Actually Has More Evidence

Order by evidence rather than by popularity and two things outside the headline four deserve more of your attention than any of them.

Broad-spectrum micronutrients have the strongest interventional evidence in this area. Rucklidge et al. (2014) randomised 80 adults with ADHD to a 36-ingredient vitamin-mineral formula or placebo for eight weeks, reporting effect sizes of 0.46–0.67 across symptoms and global functioning — though notably not on clinician-rated ADHD scales. The three-site paediatric MADDY trial found blinded clinicians rated 53–54% of children on micronutrients as improved, against 18–23% on placebo (Johnstone et al., 2022), and a blinded-clinician result is far harder to explain away than a parent-rated one. The caveats: this evidence comes largely from a single research network, independent replication is limited, and the intervention is a 36-ingredient formula, not one hero nutrient.

Vitamin D, as an add-on rather than a stand-alone, has more randomised evidence behind it than magnesium does. A meta-analysis of four trials in 256 children already taking methylphenidate found vitamin D add-on improved inattention (SMD −0.67), with a wide confidence interval across small trials (meta-analysis in Middle East Current Psychiatry, 2025). Children with ADHD also have lower serum vitamin D than controls on average (meta-analysis, 2018). It's the easiest one to act on in the UK: the NHS already advises everyone consider a daily vitamin D supplement in autumn and winter, entirely separately from ADHD.

Why Your ADHD App Won't Tell You This

Let's be direct about the incentives.

Your ADHD app makes money by keeping you subscribed. A notification timer costs them nothing. A 10-minute break reminder costs nothing. A task-breakdown algorithm costs engineering, but it scales infinitely.

Your breakfast? That doesn't scale. Every ADHD person needs different micronutrients. Testing costs money. Dosing is individual. There's no one-size-fits-all supplement to sell. And the biggest barrier to profit: food costs less than apps. A month of sardines (excellent omega-3, iron, and vitamin D) costs less than a month of their premium tier.

Meanwhile, the supplement industry profits from your ADHD impulsivity. "Brain Boost ADHD Formula™" in eye-catching packaging. $39.99. "Buy now—limited supply!" The entire marketing model is built on impulse (ironic, given your executive dysfunction). And here's the thing: most commercial ADHD supplements are underdosed, inconsistently formulated, and make claims that wouldn't survive FDA scrutiny. But the FTC mostly doesn't police supplements, because the regulatory framework is broken. The UK and EU regulate supplements as food rather than as medicines too — but the claims regime is different in kind. Only claims on the authorised EU/GB register may be made at all, which is a permitted-list system rather than a police-it-afterwards one, and it is materially stricter about what a label may say. It still won't stop a bottle being underdosed.

So you get: apps that ignore nutrition, supplement companies that exploit impulsivity, and a total information vacuum.

That's what Ecstasis is built to fix.

This is why Ecstasis tracks more than your tasks. Biology isn't optional—it's the foundation. Productivity scaffolding on a depleted brain is like pushing a car with no fuel. We're building the only ADHD app that treats you as a whole system. Join the waitlist at ecstasis.app

How to Actually Fix It (Food Before Supplements)

This is the practical part. It's boring. It's not sexy. It won't fit on an Instagram story. But it works.

Step 1: Get Tested First

This is non-negotiable. Supplementing without knowing your baseline is guessing. Some micronutrient deficiencies have overlapping symptoms. And iron overload is dangerous.

Order a micronutrient panel. Your GP (or primary care doctor) can order it, or you can use a direct-to-consumer service — Quest or LabCorp in the US, Medichecks or Randox in the UK. You need:

• Serum ferritin (iron storage) – normal is 30–300 ng/mL (µg/L — the unit UK labs typically report this in; the two are numerically identical); anything below 30 is depleted
• RBC magnesium (more accurate than serum magnesium) – normal is 4.0–6.4 mg/dL (≈1.6–2.6 mmol/L)
• Serum zinc – normal is 60–120 mcg/dL (≈9–18 µmol/L)
• Omega-3 index (optional but useful) – measured as a percentage of total red blood cell fatty acids; <4% is low

Total cost: $200–400 in the US. Most US insurance covers it if you run it through your doctor; some may require a copay. In the UK, a micronutrient panel isn't routinely available on the NHS outside of a specific clinical indication, so expect to go private if your GP won't refer you.

One caveat about that list. Serum ferritin is the test with a genuinely defensible reason to sit in an ADHD conversation — it's the one the research uses, and in the UK it's ordinary GP territory. No ADHD trial has used RBC magnesium, serum zinc or an omega-3 index to decide who should supplement; they aren't established decision tools here.

Why this matters: Konofal et al. (2004) used objective testing — serum ferritin — rather than guesswork to identify deficiency. That's the standard. Don't guess.

Step 2: Food Sources (Always Prioritise Food)

Food beats supplements in absorption, cost, and whole-system nutrition. Food has cofactors and synergists that isolated supplements don't. (Portions below use standard nutrition-database measures — ounces, cups, tablespoons; 1oz ≈ 28g if you're working in metric.)

Magnesium: pumpkin seeds (156mg per ounce), dark leafy greens (spinach, kale), almonds (76mg per ounce), dark chocolate 70%+ (50mg per ounce), wild salmon (26mg per 3oz)

Iron: oysters (5–6mg per 3oz, absorbed at 20%+ due to high vitamin C), red meat (2–3mg per 3oz), legumes (3–4mg per half-cup cooked), fortified cereals (4–18mg per serving) Pro tip: pair iron with vitamin C (orange juice, tomatoes, bell peppers) to boost absorption; avoid coffee or tea within 2 hours of iron-rich meals, as tannins block absorption

Zinc: oysters (5–7mg per 3oz), beef (5–7mg per 3oz), pumpkin seeds (2–3mg per ounce), cashews (1–2mg per ounce), chickpeas (1mg per half-cup)

Omega-3 (EPA/DHA): sardines (1,400–1,500mg EPA+DHA per 3oz can), wild-caught mackerel (1,000mg per 3oz), wild salmon (1,500mg per 3oz), walnuts (2.3g ALA per ounce—converted to EPA/DHA at low rates), flax (2.4g ALA per tablespoon)

Note: plant sources give you ALA, not EPA or DHA. ALA converts to EPA at roughly 5–10% and to DHA far less efficiently, frequently under 1% — so flax and walnuts are not DHA sources. If you eat no fish, algal oil supplies DHA directly; worth raising with your GP or pharmacist.

Step 3: Supplementation Is a Conversation, Not a Search Result

After testing. After 6–8 weeks of dietary changes. Only if you're still deficient — and then with a clinician, not a checkout page.

There are no doses here. The right answer depends on your blood results, your body weight, your medication and what else you're already taking — none of which is visible from a blog post. What we can give you is the boundaries, which supplement pages rarely carry.

• Iron — never on a hunch, and never without a ferritin result a clinician has read. Iron is the one nutrient here that can seriously harm a child, the toxic threshold depends on body weight, and it belongs behind a GP's decision. Keep supplements out of children's reach.
• Zinc — the NHS advises not taking more than 25mg a day from supplements unless a doctor recommends it, because excess zinc reduces copper absorption. That's a ceiling, not a target.
• Magnesium — the NHS position is that 400mg or less a day from supplements is unlikely to cause harm; above that, short-term diarrhoea, with long-term effects unclear. Also a ceiling, not a target.
• Vitamin B6, if you're eyeing a magnesium-B6 combination — EFSA's tolerable upper intake level is 12mg a day for adults, and combination products stack.
• Omega-3 — if you do take fish oil, brands that test for mercury and PCBs (NSF, USP verification) are worth the extra.

Every one of those is a question for your GP or pharmacist, particularly if you take ADHD medication. Take your test results to the appointment.

What to Ignore (The Supplement Industry Hype)

This is the part where we're blunt.

• "ADHD focus blends" – Overpriced, poorly dosed, rarely tested for active ingredients. Skip them.
• Nootropic stacks – Inconsistent quality, underdosed, overpromised. The "synergy" they claim often doesn't exist at that dose.
• Amazon ADHD supplements – Incentivised reviews. Inconsistent quality control. High likelihood of adulteration or under-dosing. Just don't.
• Influencer recommendations – Financial conflicts. Sponsorships. They're selling, not helping.
• Rule of thumb: if it promises "laser focus" or "instant clarity" on the packaging, it's marketing to your ADHD impulsivity, not to your biology. Skip it.

The Honest Timeline

A word on expectations.

If you're deficient and you correct it through food or supplements:

• Weeks 1–2: Probably nothing. Your brain is still adjusting.
• Weeks 3–6: Maybe subtle improvements in sleep quality, mood stability, or brain fog. Not dramatic.
• Weeks 6–12: If there's going to be an effect, this is roughly the window the trials measured in — Konofal's iron pilot ran 12 weeks, Bilici's zinc trial 12 weeks, the micronutrient trials 8. None of them produces a percentage you can apply to yourself.
• Month 4+: For omega-3 specifically, the only subgroup that reached significance in Liu et al. (2023) was studies running four months or longer. If you're trialling that one, the trial is months, not weeks.

Not magic. Modest where it's been measured at all. Clearest in people who were genuinely depleted to begin with.

And critically: where these findings exist at all, they are adjunctive — the iron, zinc and vitamin D results come from studies alongside medication, not instead of it. You're not choosing between medication and nutrition. Nobody credible is asking you to.

Your Next Step

1. Get tested. Ask your GP about a serum ferritin test. Know your baseline before you change anything.
2. Add 2–3 food sources. This week. Don't overthink it. A can of sardines on toast. A handful of pumpkin seeds. A spinach salad. Small friction.
3. Retest in 8 weeks. Same test. See what changed.
4. Take the results to a clinician. If you're still deficient after 8 weeks of deliberate food changes, that's a conversation with your GP or pharmacist — not a checkout page.

Your ADHD app won't tell you this because it's not scalable, it's not sexy, and it doesn't generate recurring revenue. But your brain knows. Your brain is chemistry. And chemistry requires raw materials.

Stop treating ADHD as a productivity problem. Start treating it as a biology problem. Apps are scaffolding. But scaffolding needs a foundation.

This is education, not medical advice

Everything above is general information to help you ask better questions — not a diagnosis, not a treatment plan, and deliberately not dosing advice. Supplements interact with medication, with each other, and with conditions you may not know you have. Never start, stop or change a supplement or medication on the basis of a blog post — talk to your GP, prescriber or pharmacist.

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References:

Arnold, L. E., & DiSilvestro, R. A. (2005). Zinc in attention-deficit/hyperactivity disorder. Journal of Child and Adolescent Psychopharmacology, 15, 619–627. PMID: 16190793.

Bilici, M., Yıldırım, F., Kandil, S., et al. (2004). Double-blind, placebo-controlled study of zinc sulfate in the treatment of attention deficit hyperactivity disorder. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 28(1), 181–190. PMID: 14687872.

Bloch, M. H., & Qawasmi, A. (2011). Omega-3 fatty acid supplementation for the treatment of children with attention-deficit/hyperactivity disorder symptomatology: systematic review and meta-analysis. Journal of the American Academy of Child & Adolescent Psychiatry, 50(10), 991–1000. PMID: 21961774.

Daubner, S. C., Le, T., & Wang, S. (2011). Tyrosine hydroxylase and regulation of dopamine synthesis. Archives of Biochemistry and Biophysics. PMID: 21176768 (PMC3065393).

EFSA Panel on Nutrition, Novel Foods and Food Allergens. (2023). Scientific opinion on the tolerable upper intake level for vitamin B6. EFSA Journal. DOI: 10.2903/j.efsa.2023.8006. PMID: 37207271.

European Commission. EU Register on nutrition and health claims. https://ec.europa.eu/food/food-feed-portal/backend/claims/files/euregister.pdf

Johnstone, J. M., Hatsu, I., Tost, G., et al. (2022). Micronutrients for attention-deficit/hyperactivity disorder in youths: a placebo-controlled randomized clinical trial. Journal of the American Academy of Child & Adolescent Psychiatry, 61(5), 647–661.

Konofal, E., Lecendreux, M., Arnulf, I., & Mouren, M. C. (2004). Iron deficiency in children with attention-deficit/hyperactivity disorder. Archives of Pediatrics & Adolescent Medicine, 158(12), 1113–1115. PMID: 15583094.

Konofal, E., Lecendreux, M., Deron, J., Marchand, M., Cortese, S., Zaïm, M., Mouren, M. C., & Arnulf, I. (2008). Effects of iron supplementation on attention deficit hyperactivity disorder in children. Pediatric Neurology, 38(1), 20–26. PMID: 18054688.

Kozielec, T., & Starobrat-Hermelin, B. (1997). Assessment of magnesium levels in children with attention deficit hyperactivity disorder (ADHD). Magnesium Research, 10(2), 143–148.

Liu, T. H., Wu, J. Y., Huang, P. Y., et al. (2023). Omega-3 polyunsaturated fatty acids for core symptoms of attention-deficit/hyperactivity disorder: a meta-analysis of randomized controlled trials. The Journal of Clinical Psychiatry, 84(5), 22r14772. DOI: 10.4088/JCP.22r14772. PMID: 37656283.

Meta-analysis of vitamin D supplementation as an adjunct in ADHD. (2025). Middle East Current Psychiatry. DOI: 10.1186/s43045-025-00586-y.

Mousain-Bosc, M., Roche, M., Polge, A., Pradal-Prat, D., Rapin, J., & Bali, J. P. (2006). Improvement of neurobehavioral disorders in children supplemented with magnesium-vitamin B6. I. Attention deficit hyperactivity disorders. Magnesium Research, 19(1), 46–52. PMID: 16846100.

NHS. Vitamins and minerals — Others. nhs.uk. https://www.nhs.uk/conditions/vitamins-and-minerals/others/

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Rucklidge, J. J., Frampton, C. M., Gorman, B., & Boggis, A. (2014). Vitamin-mineral treatment of attention-deficit hyperactivity disorder in adults: double-blind randomised placebo-controlled trial. British Journal of Psychiatry, 204(4), 306–315. PMID: 24482441.

Vitamin D levels in children with ADHD: a meta-analysis. (2018). PMID: 30367389.