Advil PM is one of the most recognizable products on any pharmacy shelf – a combination of ibuprofen for pain and diphenhydramine as a sleep aid, designed to help you rest through an aching back or pounding headache. Millions of people reach for it without a second thought. What most don’t realize is that the ingredient doing the sleep-inducing work, diphenhydramine, belongs to a drug class that researchers have linked to a meaningful increase in dementia risk, particularly with long-term or cumulative use.
Diphenhydramine, the active ingredient in Benadryl and dozens of over-the-counter sleep aids and allergy medications, is drawing serious concern from doctors and researchers who say its long-term use may significantly increase the risk of diphenhydramine dementia. Rooted in how the drug works inside the brain, this concern centers on what it does to a specific chemical system essential for memory, and what the data shows happens when someone uses it regularly over months or years.
These medications block a neurotransmitter called acetylcholine in the brain and body. Acetylcholine plays a central role in memory function, and the neurons that rely on it sit in a region of the brain directly involved in attention, learning, and recall. When a drug repeatedly damps down that system, the cumulative effect on cognitive health may be significant.
How Diphenhydramine Gets Into the Brain
First-generation H1 antihistamines cross the blood-brain barrier and impair cognitive function and psychomotor performance – the kind of impairment most people recognize as the groggy, slow-thinking feeling that follows taking a product like Benadryl or Advil PM. A 2013 study in Current Allergy and Asthma Reports found that these medications occupy more than 70% of the CNS H1 receptors (the brain’s histamine receptors), which is why the sedation is so pronounced and why the cognitive effects go well beyond mild drowsiness.
Basal forebrain cholinergic neurons are essential for cognitive processes, including attention, learning, and memory. As the brain ages, this system naturally becomes less robust. Diphenhydramine blocks acetylcholine, lingers in older bodies for up to 18 hours, and contributes to a cumulative anticholinergic burden that compounds with every additional medication in the same class. Many older adults take multiple drugs with anticholinergic (acetylcholine-blocking) properties simultaneously, stacking the effect without realizing it.
A nested case-control study published in JAMA Internal Medicine, involving 58,769 patients diagnosed with dementia and 225,574 matched controls, found statistically significant associations of dementia risk with exposure to anticholinergic antidepressants, antiparkinson drugs, antipsychotic drugs, bladder antimuscarinics, and antiepileptic drugs. Antihistamines like diphenhydramine sit within that same broader anticholinergic class.
What the Research Shows on Dementia Risk
A 2015 study published in JAMA Internal Medicine found that people who took diphenhydramine daily for at least three years had a 54% higher risk of developing dementia than people who took the same dose for three months or less. That study, led by Dr. Shelly Gray of the University of Washington School of Pharmacy, followed more than 3,400 older adults over a decade, making it one of the most cited pieces of evidence in this area.
The National Institute on Aging noted that older adults who take anticholinergic drugs may be at significantly higher risk of developing dementia, with the highest cumulative users showing the 54% elevated risk. The risk of Alzheimer’s disease alone was raised by 63 percent at the highest anticholinergic doses, according to Neurology Today.
More recent research has continued to build on this picture. A 2024 study published in the Journal of Allergy and Clinical Immunology: In Practice looked specifically at patients with allergic rhinitis and found that those taking first-generation antihistamines faced escalating dementia risk with increasing cumulative dosage. First-generation antihistamines like diphenhydramine showed a higher risk profile than second-generation alternatives.
A 2025 correspondence published in the same journal extended that finding further. Researchers found that long-term use and increasing cumulative doses of antihistamines may be associated with a higher risk of developing dementia, reinforcing the pattern seen in earlier work. And in a large population-based analysis of nursing home residents, use of anticholinergic medications was associated with a 26% increase in dementia risk among 141,740 elderly nursing home residents with depression.
The majority of these studies are observational, meaning they identify associations rather than prove direct causation. People who take sleep aids regularly may also have other underlying health conditions, sleep disorders, or factors that independently affect dementia risk. Researchers have worked to control for these variables, but no observational study can fully eliminate the possibility of confounding. What the body of evidence does show, consistently, is a dose-dependent relationship: the more diphenhydramine taken, and for longer, the higher the apparent risk.
Why Older Adults Are at Greater Risk
The aging brain is more vulnerable to anticholinergic drugs for a specific reason. As we get older, the body naturally produces less acetylcholine. Medications like diphenhydramine that further suppress this already-reduced supply hit harder and last longer in an older system. Diphenhydramine is considered a high-risk medication for the elderly, as it may increase the risk of falls – a consequence of both impaired coordination and prolonged sedation. The sedating effects significantly increase fall risk, with drowsiness, dizziness, and impaired coordination persisting well beyond the medication’s intended duration.
There’s also the matter of how the body processes the drug. Older adults frequently have reduced kidney and liver function, meaning the drug stays active in the bloodstream longer than it would in a younger person. OTC sleep medications containing diphenhydramine increase the risk of hepatic and renal insufficiency, drug interactions, and adverse events in the elderly.
Research from the University of Washington found that people taking at least 4 mg per day of chlorpheniramine – another first-generation antihistamine in the same class as diphenhydramine – for more than three years would be at greater risk for developing dementia. That dosage threshold, while specific to chlorpheniramine, reflects the same cumulative pattern researchers have identified across this entire drug class, even though the exact equivalent for diphenhydramine continues to be studied.
The Difference Between First- and Second-Generation Antihistamines
Second-generation H1 antihistamines cross the blood-brain barrier to a significantly smaller extent than their predecessors, which is why medications like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) are associated with far less sedation and fewer cognitive effects.
Newer second-generation antihistamines such as cetirizine, loratadine, and fexofenadine have little to no evidence linking them to dementia and are considered much safer for the brain than older allergy medicines, according to myALZteam, a resource for people affected by Alzheimer’s disease. That said, the 2025 research in the Journal of Allergy and Clinical Immunology: In Practice noted that long-term and high-dose use of even second-generation antihistamines warrants ongoing monitoring, though current evidence places them well below first-generation drugs in terms of cognitive risk.
For people who need something for sleep specifically, the diphenhydramine route is increasingly being questioned by clinicians. Major allergy and geriatric guidelines now consistently list diphenhydramine as a drug to avoid in older adults. The American Geriatrics Society recommends avoiding Benadryl in seniors, and the same principle extends to other diphenhydramine-containing products, including Advil PM, Tylenol PM, ZzzQuil, and generic nighttime sleep formulas.
For those dealing with both pain and sleep problems, talking to a doctor about alternatives is the practical next step. Non-anticholinergic sleep options exist, including low-dose melatonin (which has no anticholinergic properties) and short-course use of other agents under medical supervision. For allergies, the switch to a second-generation antihistamine taken once daily is straightforward and, for most people, equally effective.
You can also learn more about other medications linked to cognitive decline that may be hiding in plain sight in your medicine cabinet.
What to Do Now
Diphenhydramine dementia risk does not scale from a single tablet taken occasionally. Taking a Benadryl once a month for an allergic reaction is not a cause for alarm, but the risk increases significantly when these drugs become a daily ritual for months or years. The research consistently points to cumulative dose as the critical variable – occasional use sits in a very different risk category from chronic nightly use.
For anyone over 55 who currently relies on diphenhydramine-containing products for sleep or allergies more than a few times per month, the most useful step is a direct conversation with a doctor or pharmacist about alternatives. Safer antihistamine options like cetirizine or loratadine are specifically recommended for managing allergy symptoms in older adults. For sleep, ask about options that don’t carry anticholinergic properties. And if you’re already taking multiple medications, ask your pharmacist to check your total anticholinergic burden – the combined effect of all the medications you take that block acetylcholine. That cumulative picture is what researchers say matters most.
Disclaimer: The author is not a licensed medical professional. The information provided is for general informational and educational purposes only and is based on research from publicly available, reputable sources. It is not intended to constitute, and should not be relied upon as, medical advice, diagnosis, or treatment. Always consult a licensed physician or other qualified healthcare provider regarding any medical condition, symptoms, or medications. Do not disregard, avoid, or delay seeking professional medical advice or treatment because of information contained herein.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
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