The Drugs Nobody Counts
Brain fog is a symptom, not a diagnosis. The most reversible cause of it is usually sitting in a bathroom cabinet, bought without a prescription.
Apatient arrived convinced she had early Alzheimer’s. She was forgetting words, rereading the same paragraph, losing the thread of her own sentences. Her scans were clean and her bloods were normal.
What she had been doing, every night for two years, was taking a PM painkiller to get to sleep. She had never written it on a medication list, because nobody had prescribed it. To her it was not a drug. It was just the thing in the cabinet.
What the pill in the cabinet actually does
The active ingredient in most over the counter sleep aids is an old antihistamine, and it is also what the letters PM stand for on a painkiller box. It makes people drowsy, which is why it ended up in sleeping pills.
Drowsiness is not the interesting part. The same drug also blocks acetylcholine, which is a separate job in a separate system. Block that dial and you do not lose what you already know, you lose the ability to lock on and to lay anything new down. That is the texture of fog: reading the same paragraph three times, not amnesia.2
The two effects travel in one pill and get confused constantly. The sleepiness is mostly the antihistamine half, while the trouble with attention is the acetylcholine half.
The headline everybody has heard
Ten years ago a study linked heavy long term use of these drugs to dementia, and that headline has circulated ever since. It deserves a proper look rather than a repeat.3
In July of this year the UK medicines regulator finished reviewing the question for the bladder drugs in the same family. It read seventeen studies, judged every one at serious or critical risk of bias, and concluded the evidence is not sufficient to confirm that these drugs raise dementia risk, while adding that a small increase cannot be ruled out. Almost nobody reported it.5
Two further things are rarely mentioned. The largest study on record found no significant signal for antihistamines specifically, and because it read prescription records it could not see the drug this whole panic is about, which you buy off a shelf.4
| Year | Study | Finding |
|---|---|---|
| 2000 | Weiler et al. Ann Intern Med | Forty drivers in a simulator. One 50 mg dose produced the worst driving of four conditions on the main task. Funded by the maker of a rival drug, which came out clean. |
| 2015 | Gray et al. JAMA Internal Medicine | The study that started the scare. People whose heavy use was entirely in the past carried the same raised risk as current users. |
| 2019 | Coupland et al. JAMA Internal Medicine | Nearly 59,000 dementia cases. Antihistamines alone were not statistically significant, and the database cannot see anything bought over the counter. |
| 2026 | MHRA Public assessment report | Seventeen studies, all at serious or critical risk of bias. Not enough to confirm the risk. A small one cannot be ruled out. |
These do not disagree. Three are observational and one is a regulator reading all of them at once. The only randomised result in the table is the first.
The unit of harm is not one drug. It is the stack, and nobody owns the stack.
- It never goes on the list Bought off a shelf, taken nightly, and not counted as a medicine by the person taking it.
- The prescriptions arrive Something for nerve pain, something for muscle spasm. Both sit in the same family, and almost no patient has been told that.
- A hospital stay adds more A patch for nausea, something else for the breakthrough. Each one sensible on its own.
- Four or five inside two days In a seventy four year old, after an operation, and not one of them counted as anticholinergic by anybody in the chain.
- Then the family asks what happened Nobody had the list. The fix is counting, and counting costs nothing.
Where this gets ahead of the evidence
“These drugs cause dementia.”
Not confirmed
Every study behind that claim is observational, and the regulator that reviewed them all this year would not confirm it. A small risk has not been ruled out either.5
“Then it is safe, and I can ignore this.”
No
The opposite. The short term effect is the one that was actually randomised, and it is large. Feeling only mildly drowsy tells you very little about how impaired you are.1
“So I should stop taking mine.”
Ask yours
Not on the strength of a newsletter. Some drugs in this family cause real trouble if stopped suddenly. The job is to produce the list, not to edit it alone.
The fog is real, the dementia headline is not what proves it, and the most reversible cause I see is the one nobody wrote down.
References
- Weiler JM, Bloomfield JR, Woodworth GG, et al. Effects of fexofenadine, diphenhydramine, and alcohol on driving performance. Ann Intern Med, 2000
- Tashiro M, Duan X, Kato M, et al. Brain histamine H1 receptor occupancy of orally administered antihistamines measured by positron emission tomography. Br J Clin Pharmacol, 2008
- Gray SL, Anderson ML, Dublin S, et al. Cumulative use of strong anticholinergics and incident dementia: a prospective cohort study. JAMA Internal Medicine, 2015
- Coupland CAC, Hill T, Dening T, et al. Anticholinergic drug exposure and the risk of dementia: a nested case-control study. JAMA Internal Medicine, 2019
- Medicines and Healthcare products Regulatory Agency. Bladder anticholinergics: review of risk of dementia. Public assessment report, 27 July 2026. gov.uk
Figure. Acetylcholine pathway, by BruceBlaus, licensed CC BY-SA 4.0, via Wikimedia Commons.