Is it ADHD, or something else?

Why everything looks like ADHD

Trouble concentrating, restlessness, forgetfulness, a mind that won’t settle, reacting too strongly to small things — none of these belongs to ADHD alone. They’re the shared surface of a whole cluster of conditions. That’s why reading about ADHD can feel like a mirror even when something else is going on, and also why it can feel like a mirror when ADHD is going on alongside two other things.

The honest framing up front: for adults with ADHD, having at least one other diagnosis is more common than not. The US National Comorbidity Survey Replication (Kessler et al.)⁴ found adults with ADHD carried sharply elevated rates of anxiety, mood, and substance-use disorders. So the question is rarely “ADHD or anxiety?” as a clean either/or. More often it’s “ADHD, anxiety, both, or anxiety that’s actually downstream of unmanaged ADHD?” — and that’s a clinical judgement, not a self-test. DSM-5-TR builds the overlap problem right into the ADHD criteria with a clause requiring the symptoms not be better explained by another disorder (DSM-5-TR)¹. Working through that clause is the differential.

What follows is what a clinician weighs when they sort it out. It is not a checklist for deciding your own case. If you recognise yourself in several of these, that’s expected — overlap is the whole point — and it’s the reason the sorting takes a trained person with your full history in front of them.

Anxiety

Anxiety and ADHD share a lot of visible territory: restlessness, a racing mind, trouble concentrating, difficulty sitting still, sleep that won’t come. Someone with generalised anxiety and someone with ADHD can describe the same Tuesday in nearly the same words.

The distinguishing question a clinician asks is what the attention problem is tied to. In ADHD, the inattention is lifelong and pervasive — it shows up across school, work, relationships, hobbies, and quiet moments, and it was there long before any particular stress. In anxiety, the concentration problem tends to track the worry: it spikes with a looming deadline or a feared situation and eases when the threat passes. ADHD attention scatters even when nothing is wrong; anxious attention is captured bythe thing that’s wrong.

Two complications keep this from being clean. First, the two genuinely co-occur a lot (Katzman et al.)⁵. Second — and this is the one that gets missed — anxiety can be downstreamof untreated ADHD. Years of missed deadlines, forgotten commitments, and last-minute scrambling teach a nervous system to brace. The worry is real, but it’s a reaction to a life that keeps going sideways, and treating only the worry leaves the engine running. A clinician who finds anxiety that started in adolescence or adulthood, layered on top of a lifelong attention pattern, will ask which one is driving.

Depression

Depression overlaps with ADHD on concentration, motivation, fatigue, and a sense of not being able to start anything. Low drive and a foggy head read the same from the outside whichever condition is producing them.

The separation is mostly about shape over time. Depression usually comes in episodes — a stretch of weeks or months that’s clearly different from your baseline, often with a low or flat mood, loss of interest in things you normally enjoy, and changes in sleep and appetite that move together. ADHD is a trait, not an episode: the attention and follow-through problems are roughly constant across your life, not a window that opened last spring. Clinicians also watch whether the symptoms are mood-congruent— in depression the difficulty concentrating travels with the low mood and lifts when the mood lifts, whereas in ADHD it’s there in good moods and bad.

As with anxiety, depression can be both a separate condition and a consequence. A person who has spent twenty years feeling like they’re underperforming for reasons they can’t name has a plausible path into depression. The clinician’s job is to see whether the depressive episodes sit on top of a lifelong ADHD pattern or explain it on their own.

Bipolar — the one to get right

This is the differential that matters most, for a practical reason: stimulant medication, the first-line treatment for ADHD, can destabilise an undiagnosed bipolar disorder — pushing someone toward mania or rapid mood cycling. Getting this one wrong isn’t a matter of treating the wrong thing slowly; it can make the person actively worse (Asherson et al.)⁶. So clinicians treat the bipolar screen as non-optional before any stimulant decision.

The overlap is real. Bipolar disorder and ADHD can both involve distractibility, fast or pressured talking, impulsivity, racing thoughts, and trouble sleeping. The distinguishing feature is episodic versus constant. Bipolar disorder cycles: there are discrete periods of elevated or irritable mood (mania or hypomania) and, usually, periods of depression, each lasting days to weeks and clearly departing from the person’s usual state. During a manic or hypomanic period the energy, grandiosity, reduced need for sleep, and risk-taking come as a package that switches on and later switches off. ADHD has no such on/off cycle — the restlessness and impulsivity are a steady baseline, present in the same form last month, last year, and at twelve.

A clinician will ask specifically about distinct mood episodes: a few days of needing almost no sleep yet feeling wired and productive, spending sprees, talking so fast people couldn’t keep up, feeling unusually powerful — periods that family noticed as out of character. A “yes” there changes the treatment plan entirely, often toward a mood stabiliser before any conversation about stimulants. The two can also coexist, which is exactly why the screen is structured and not skipped.

Trauma and PTSD

Trauma — and post-traumatic stress disorder (PTSD) specifically — overlaps with ADHD on hypervigilance that looks like restlessness, concentration that won’t hold, irritability, sleep problems, and emotional reactions that feel bigger than the moment calls for (ADHD/PTSD overlap review)⁷. A trauma-shaped nervous system that’s always scanning for danger can be hard to tell from an ADHD-shaped one that can’t hold a thread.

The separator is onset and tie to events. PTSD symptoms have a beginning — they follow a traumatic experience or a period of them, and they often come with intrusions specific to it: flashbacks, nightmares, avoidance of reminders, a sense of reliving. ADHD has no originating event; it’s present from childhood, before anything happened. When a clinician maps the timeline and finds the attention and dysregulation problems predate the trauma, that points one way; when they began after it, that points another. Complex or developmental trauma muddies this, which is part of why the history-taking is careful rather than quick, and why the two are frequently found together rather than instead of each other.

Autism and AuDHD

Autism and ADHD overlap and co-occur often enough that the combination has its own informal name, AuDHD (autism/ADHD comorbidity review)⁸. Both can involve difficulty with focus that’s actually a focus in the wrong place, sensory sensitivity, trouble with transitions, social exhaustion, and executive-function struggles around planning and starting tasks. Someone can read as “definitely ADHD” and have the autism missed, or the reverse, or have both.

Distinguishing features lean on social and pattern-related traits more than attention: autism brings differences in social communication, a strong need for sameness and routine, intense focused interests, and sensory profiles that are central rather than incidental. ADHD’s attention is variable and novelty-seeking; autistic focus is often deep and sustained on specific interests. Because they frequently coexist, the clinical question is usually “is autism also present?” rather than “which one is it?”. For how this presents in adult women and in people diagnosed late — where both are routinely overlooked — see AuDHD in women.

Medical mimics

Some look-alikes aren’t psychiatric at all, and a careful workup rules them out before settling on ADHD (APSARD 2024)². The recurring ones:

• Sleep disorders.Chronic sleep deprivation, sleep apnoea, and circadian-rhythm problems produce inattention, irritability, and a foggy, can’t-focus state that mirrors ADHD — and ADHD itself disrupts sleep, so the relationship runs both ways. See sleep and ADHD.
• Thyroid dysfunction.An under- or over-active thyroid can cause concentration problems, restlessness, fatigue, and mood changes. It’s a cheap blood test and a standard part of the rule-out.
• Perimenopause. Falling and fluctuating oestrogen brings brain fog, word-finding trouble, and concentration problems that arrive in midlife and get misread as new-onset ADHD — or that unmask ADHD that was always there. See perimenopause and ADHD.

None of these means you don’t have ADHD. They mean a clinician checks them off rather than assuming, because if one of them is the whole story, ADHD treatment won’t fix it.

Why getting it right matters

Two failure modes show up often enough to name. The first is the bipolar one above: a stimulant prescribed for what looked like ADHD, landing on an undiagnosed bipolar disorder and triggering mania. That’s the reason the mood-episode screen isn’t a formality.

The second is quieter and more common. A person is treated for anxiety or depression for years — therapy, an antidepressant, real effort — and gets partial relief at best, because the anxiety or low mood is being generated by an untreated ADHD underneath. They’re anxious because their life keeps falling apart in ways they can’t control; they’re low because they’ve concluded they’re lazy or broken. Treating the surface condition without seeing the ADHD driving it can mean a decade of not-quite-working treatment. This is one reason late ADHD diagnoses so often arrive through a mental-health door that was opened for something else.

The flip side also holds: ADHD-style emotional reactivity can itself be mistaken for a mood disorder. How that intensity actually works in ADHD — and how it differs from a depressive or bipolar mood — is its own topic in ADHD and emotional regulation.

What the clinician actually does

Running the differential is a structured process — most of what a real assessment is (NICE NG87)³:

• A structured developmental history.Not just “how are you now” but the whole arc — childhood, school, work, relationships — looking for where each pattern started.
• Age of onset. ADHD requires evidence of symptoms before age 12. A pattern that genuinely began in adulthood points away from ADHD and toward something else, which is why clinicians press on early history and collateral from people who knew you young.
• Pervasiveness. ADHD shows up across settings, not in one stressful corner of life. Symptoms confined to a single situation read more like a response to that situation.
• A comorbidity and mood-episode screen. Explicit questions about anxiety, depression, and — critically — distinct periods of mood elevation, because the bipolar answer changes treatment.
• The medical mimic rule-out. Thyroid, sleep, and other physical contributors checked rather than assumed away.

The output of that process is often “ADHD, and also…” rather than “ADHD, not the other thing.” If you walk in recognising yourself in half of this article, that’s ordinary, and it’s precisely the situation the structured assessment exists to sort out. For what that assessment looks like and how to prepare for it, see getting diagnosed as an adult.