ADHD or Long COVID brain fog? What separates them

Since the pandemic, many adults have noticed that their thinking feels slower, foggier or more scattered than before. Some wonder whether they have had ADHD all along, now unmasked. Others wonder whether an infection changed their brain. Both questions are reasonable, and the answer starts with timing. This page compares Long COVID brain fog with adult ADHD and explains what research has found so far.

What Long COVID is

The Centers for Disease Control and Prevention (CDC) defines Long COVID as a chronic condition that occurs after infection with SARS-CoV-2, the virus that causes COVID-19, and is present for at least three months. CDC says symptoms can emerge, persist, resolve and reemerge over weeks, months or even years, and that anyone who had the infection can develop it, though it occurs more often after severe illness.

CDC lists more than 200 identified symptoms. It says fatigue, brain fog and post-exertional malaise (symptoms that get worse after physical or mental effort) are commonly reported. Its list of neurological symptoms includes difficulty thinking or concentrating, which it notes is sometimes called "brain fog," along with headaches, sleep problems, dizziness on standing, pins-and-needles feelings, changes in smell or taste, and depression or anxiety.

"Brain fog" is an everyday phrase, not a formal diagnosis. It describes an experience, slowed or cloudy thinking, that can have many causes.

What research shows about thinking after COVID-19

Two large studies help put the complaint in context.

• NIH RECOVER cohort (JAMA, 2023). Researchers followed 9,764 adults at 85 sites in 33 states, Washington, DC and Puerto Rico. Brain fog was one of 12 symptoms that contributed to their Long COVID score, along with post-exertional malaise, fatigue, dizziness, palpitations and loss of smell or taste. Among 2,231 people first infected on or after December 1, 2021 and enrolled within 30 days, 10% met the Long COVID definition at six months.
• Cognition testing in England (New England Journal of Medicine, 2024). Researchers at Imperial College London gave online cognitive tests to 112,964 adults. People whose COVID-19 symptoms had resolved showed small deficits compared with people who had not been infected. People with unresolved persistent symptoms showed larger deficits, with the biggest gaps on memory, reasoning and executive function tasks. Those test results correlated only weakly with people's own reports of brain fog, and the authors said long-term persistence remains uncertain.

Memory, reasoning and executive function are areas where many adults with ADHD also struggle, which is part of why the two get compared.

The key differences

Sources for the table: NIMH's adult ADHD publication for the ADHD column; CDC's Long COVID basics and signs pages, plus the 2024 NEJM study for measured deficits, for the Long COVID column.

Timing is the main clue

NIMH states that adults diagnosed with ADHD must have shown symptoms starting before age 12, so an ADHD evaluation leans heavily on childhood and school history. If you focused well through school and work and the trouble began in the weeks or months after an infection, that history points away from ADHD and toward Long COVID or another medical cause. If you can trace restlessness, forgetfulness and disorganization back to elementary school, and they simply feel worse lately, ADHD stays on the table.

A few other questions help. Does the fog get worse after a busy day or a workout, and take a day or more to lift? CDC describes that pattern of symptoms worsening after physical or mental effort. Are there physical symptoms alongside the thinking problems? Is sleep worse than before? CDC lists sleep problems as a Long COVID symptom, and poor sleep affects attention in its own right; see ADHD and sleep problems and ADHD or sleep apnea.

Can both be present?

Yes. CDC says anyone who has had COVID-19 can develop Long COVID, and that includes adults who already have ADHD. An adult with lifelong ADHD who develops new fatigue and fog after an infection may need both conditions considered. Depression and anxiety, which CDC lists among Long COVID symptoms, can also affect concentration; the ADHD or depression and ADHD or anxiety pages explain those comparisons.

What to do with new brain fog

CDC advises talking to a healthcare provider if you have symptoms that last for months after an infection. A positive COVID-19 test is not required for a Long COVID diagnosis, according to CDC; providers consider health history, a prior diagnosis based on a test, symptoms or exposure, and a health examination. A primary care clinician is usually the right first stop for a medical workup.

If an ADHD evaluation is also on your mind, bring a clear timeline: when you were infected, when the thinking problems began, and what your attention and organization were like in childhood. NICE guidance for ADHD says diagnosis requires a full clinical assessment with developmental history and attention to coexisting conditions and physical health. Dr. Shershow's evaluation covers current symptoms, childhood and work history, medical history and medications, sleep, and other explanations for symptoms, then explains what it found and which options fit. See can you develop ADHD as an adult? for more on adult-onset symptoms, and what if it's not ADHD for next steps if another cause is found.

When to talk to a clinician

Talk to a clinician if thinking problems have lasted for months, interfere with work or daily life, or come with fatigue or physical symptoms. Start with a medical provider if the problems began after an illness. If you suspect a lifelong attention problem, the adult ADHD self-test can help you organize what you notice, though it cannot tell ADHD from Long COVID. Adults in New York or New Jersey can read ADHD testing in NYC or ADHD testing in New Jersey to see how an evaluation with Dr. Shershow works.