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October 22, 2021

Parkinson's disease is a chronic, progressive neurological disease, characterized by the drastic reduction of dopamine transporters and the dopaminergic neurons upon which they are expressed. The resulting symptoms include bradykinesia (slowness of initiation of voluntary movements), tremors, rigidity, and postural instability.
Taiwan's National Health Service covers about 99 percent of its 24 million inhabitants, and maintains complete records in its National Health Insurance Research Database. The Longitudinal Health Insurance Database2000 (LHID 2000) is a nationally representative subset of the latter.
Using the LHID 2000, a Taiwanese research team identified10,726 patients with Parkinson's disease. It paired them with an identical number of randomly selected non-Parkinson's controls, matched by age, gender, and index date (first date of diagnosis of Parkinson's disease).
The team then looked retroactively through the database to determine which of the 21,452 individuals had previously been diagnosed with ADHD. Fourteen of the 10,726 Parkinson's patients had been diagnosed with ADHD, versus five of the 10,726 in the control group.
Parkinson's patients were thus 2.8 times as likely to have had a previous diagnosis of ADHD as the controls. When adjusted for age, gender, and Carlson Comorbidity Index scores, they were 3.6 times as likely to have had a previous ADHD diagnosis.
The authors cautioned that this association between prior ADHD diagnosis and subsequent Parkinson's diagnosis is not causal.
Only one in 766 of Parkinson's patients (a seventh of one percent) had previously been diagnosed with ADHD. So even if there were any causal relationship, it would be extremely weak.
Huang-Chuen Fan, Yu-Kang Chang, Jeng-Dau Tsai, Kuo-Liang Chiang, Jui-Hu Shih, Kuan-Yi Yeh, Kuo-Hsing Ma, and I-Hsun Li, "the association Between Parkinson's Disease and Attention-Deficit HyperactivityDisorder," Cell Transplantation (2020) 29, 1-13, https://doi.org/10.1177/0963689720947416.
Language is powerful. The words we choose not only reflect our understanding of the world but also actively shape it. Recently, this truth has been at the center of a growing debate in the mental health field regarding how we talk about ADHD.
In a recent paper published in The Lancet Psychiatry titled “The Power of Words: Respectful Language in ADHD Research,” French and colleagues advocated for a shift toward "neurodiversity-affirmative language”. Rooted in the social model of disability, their proposal encourages researchers to abandon traditional medical terminology, e.g., words like disorder and deficit, in favor of more neutral terms such as condition and challenge.
My colleague, Dr. Michael Miller, and I read this with great interest. We completely agree that revising language is essential to good science and that, both as researchers and as human beings, we are ethically bound to speak respectfully. However, we felt compelled to write a response. In our new paper, we argue that while language must evolve, it must do so scientifically.
The Two Prerequisites for Language Change
If we are going to fundamentally shift our scientific lexicon, two requirements must be met:
Currently, the proposal by French and colleagues meets neither requirement. While they claim consensus is accumulating that certain terms are disrespectful, they provide zero empirical evidence that this view is shared by the community of individuals living with ADHD. Even proponents of patient-centered language admit there is surprisingly little data supporting specific language changes.
More alarmingly, the recommended changes severely dilute the scientific accuracy of our field. Let’s look at two examples.
Why a "Deficit" is Not Just a “Challenge"
French and colleagues suggest replacing the term deficit with challenge. On the surface, challenge sounds softer and more affirming. But scientifically, these words are not interchangeable.
For decades, the term deficit has been defined by a specific performance metric that falls substantially below an expected level. It is a measurable reality. A challenge, on the other hand, refers to a new or difficult task that tests someone's ability.
Every single human being is "challenged" by complex neuropsychological tests, but only some individuals who face that challenge demonstrate scientifically significant deficits. If we relabel measurable deficits as universal challenges, we sacrifice the exactness required to communicate scientific findings and accurately measure the effects of life-changing treatments.
ADHD is a Disorder, Not Just a "Condition"
Another proposal is to replace the word disorder with condition.
In mainstream psychiatry, a disorder is a clinically significant disturbance that causes distress or disability. The word purposefully separates natural human variation from the suffering (pathos) that gives pathology its meaning.
Condition is a completely neutral term. Pregnancy is a condition. Being tall is a condition. Calling ADHD a condition distances the diagnosis from the profound suffering it can cause.
French et al. argue against framing ADHD as a disorder because it exists on a spectrum without a clear cutoff, its manifestation is context-dependent, and its definition evolves. But if we apply that logic across all of medicine, the concept of disease unravels:
The Real-World Danger of Imprecise Language
This is not merely an academic debate over semantics. The language we use has real-world implications. In the United States and across the globe, our healthcare, educational, and legal systems run on precise medical language. Terms like impairment, dysfunction, and disorder are legally and administratively required to justify support services, workplace accommodations, specialized educational therapies, and medications. The language of pathology in diagnostic manuals regulates the flow of these resources.
If we reclassify ADHD as a neutral condition characterized only by challenges, we risk erecting massive bureaucratic barriers. Imprecise language could easily be used by institutions or insurance companies to deny vital care to the people who need it most.
The Need for Lexical Discipline
Attempting to characterize a clinical disorder entirely through its strengths happens in a scientific vacuum. We cannot ignore the vast body of rigorous evidence confirming that ADHD meets the long-standing criteria used by mental health science to identify clinical disorders.
As professionals, our respect for the ADHD community demands a commitment to language that is clear, correct, and evidence-based. To build genuine consensus about how we talk about ADHD, we need meaningful, collaborative dialogue that integrates compelling empirical data and rigorous theory.
This standard of "lexical discipline" is not just a technical preference. It is a vital mechanism through which science and the mental health professions uphold their duty to society.
For many ADHD patients, getting properly diagnosed and starting meds is only half the battle. The next step is figuring out the exact right dose. Historically, clinical guidelines have provided scant guidance on this critical step. This lack of direction can inadvertently foster two extremes in clinical practice: therapeutic inertia (settling for a subtherapeutic dose that leaves symptoms undertreated) or uncritical escalation (driving doses higher and higher beyond licensed limits without meaningful benefit).
To clear up this pharmacological gray area, an international team of researchers published the first comprehensive dose-effect network meta-analysis of ADHD medications in The Lancet Psychiatry. By pulling together a massive vault of clinical trial data, they mapped out exactly how efficacy and tolerability shift as doses increase.
Traditional meta-analyses evaluate head-to-head, pairwise data, comparing one drug at a specific dose directly against a placebo. However, this study utilized an advanced Bayesian hierarchical network model using restricted cubic splines.
This mathematical framework allowed the researchers to combine both direct trial data and indirect evidence simultaneously across 113 double-blind randomized controlled trials (RCTs). In total, the study evaluated data from 14,138 children/adolescents and 11,016 adults. By standardizing various formulations into basic equivalents (e.g., converting amphetamines to dextroamphetamine equivalents), they created a clear, unified map of dose ranges.
The study yielded distinct dose-response curves depending on the patient's age and the specific medication class. Rather than a linear trend in which "more medicine equals more benefit," most treatments reach a clear statistical plateau or ceiling.
For Children and Adolescents (under 18)
In the pediatric population, medications hit clear peak efficacy boundaries:
For both amphetamines and guanfacine, escalating the dosage past these points resulted in U-shaped curves, meaning further dose hikes yielded diminishing group-level symptom reduction.
For Adults (18 and older)
Adult profiles showed slightly different trajectories:
The ultimate goal of this landmark analysis is to guide shared decision-making between clinicians, patients, and families. The results send a dual message to the medical community:
A medication's true efficacy hinges on its tolerability, typically measured by how often patients discontinue treatment due to severe side effects. For amphetamines, this dropout risk scales linearly with dosage, notably exceeding placebo in children above 25 mg/day and becoming prominent in adults past 50 mg/day. In contrast, methylphenidate shows no clear dose-dependent dropout risk in pediatric patients, whereas adults face a steep risk curve: increasing the dose from 60 mg/day to 90 mg/day raises the dropout risk from 7.3% to 10.0% for only modest symptom relief. Finally, youth taking guanfacine experience a sharp climb in discontinuation risks, reaching a 9.8% median risk at 4 mg/day before data limitations obscure further trends.
The authors strongly emphasize that these findings represent group averages. Because individual metabolism, genetics, and comorbidities vary widely, some specific patients may legitimately require and tolerate higher off-label doses. However, if an unusually high dose is needed, the study suggests it should prompt a careful clinical pause, either to reassess for co-occurring conditions (like anxiety, autism, or sleep disorders) or to manage realistic expectations regarding what the medication can achieve.
The persistent shortage of ADHD medications has been more than a simple annoyance for patients at the pharmacy; the inconsistent availability of these medications has had deep impacts on the daily lives of those struggling without them. While public discourse has pointed fingers at over-prescribing or at restrictive DEA quotas, a recent economic evaluation in JAMA Health Forum suggests we’ve been looking in the wrong direction for an answer to what is causing this.
The reality of the shortage is less about increased demand and more about a fragile, globalized supply chain that snapped at a critical link.
Debunking the "Quota Myth":
The prevailing narrative suggested that the Drug Enforcement Administration (DEA) was stifling production by refusing to raise quotas. However, the data tells a different story. In 2022, manufacturers collectively met only about 70% of their allotted production quotas.
So we know that the problem wasn't that this DEA quota ceiling was too low. In fact, most manufacturers couldn't even reach it. Even when accounting for exports and domestic retail, production remained significantly below the legal limit. Even if the DEA had doubled its quotas, these medications still likely wouldn't have magically appeared on pharmacy shelves.
The most striking finding in the study is the correlation between the shortage and a sharp decline in the import of raw Active Pharmaceutical Ingredients (APIs). For the past decade, Germany has accounted for over 85% of US amphetamine imports. In 2022, these imports dropped by approximately 36.7%. When the API doesn't arrive at the factory, production for medium and small manufacturers grinds to a halt. Unlike larger pharmaceutical giants, these smaller players often lack the inventory cushion or flexibility to quickly pivot to a new supplier.
When the primary supply of amphetamine-based stimulants (like Adderall) faltered, it triggered a secondary crisis. Patients and clinicians, seeking alternatives, shifted toward lisdexamfetamine (Vyvanse) and methylphenidate (Ritalin/Concerta).
If we view this shortage purely through a regulatory or clinical lens, we miss the underlying cause of the crisis. The pharmaceutical industry has become a victim of its reliance on "just-in-time manufacturing” and highly concentrated sourcing. Because over 30% of APIs for the US market are produced in just one or two facilities globally, the system isn't just inefficient; it’s brittle. We are, in a sense, trapped in a system that prioritizes cost-reduction over the resilience required for public health.
The researchers suggest several policy shifts to prevent a repeat of this supply chain failure:
The ADHD medication shortage wasn't a failure of clinical oversight or a sudden surge in "TikTok-driven diagnoses”, as many have suggested. It was a failure of logistics. It reminds us that the path from a lab in Germany to a patient's hand in the US is far more precarious than we realized.
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