Anticholinergic effects are the collection of symptoms that arise when something blocks acetylcholine, a chemical messenger your body uses to control everything from saliva production to memory formation. These effects range from dry mouth and blurred vision to confusion and, with prolonged exposure, a measurably higher risk of dementia. The causes are surprisingly varied: hundreds of prescription and over-the-counter medications carry some degree of anticholinergic activity, and even certain plants produce natural compounds that block the same pathways.
What Acetylcholine Does and Why Blocking It Matters
Acetylcholine is one of the most widely distributed chemical messengers in the body. In the brain, it modifies how neurons fire and communicate, shaping your ability to pay attention, form memories, and stay alert.1PubMed Central. Acetylcholine as a neuromodulator: cholinergic signaling shapes nervous system function and behavior Outside the brain, it governs a long list of automatic functions: it tells your salivary glands to produce saliva, your gut muscles to push food along, your bladder to contract, your sweat glands to cool you down, and your pupils to constrict in bright light. It operates through two broad families of receptors, called muscarinic and nicotinic, which are distributed differently across the body.2PubMed Central. Acetylcholine in Brain-Body Communication: Biological Mechanisms and Physiological Roles
When a drug or substance blocks acetylcholine at these receptors, the systems that depend on it slow down or stop working properly. Most drugs that people call “anticholinergic” are really blocking the muscarinic type of receptor, which accounts for the classic pattern of symptoms most patients encounter. The nicotinic receptors are a separate story: the classic muscarinic blocker atropine, for instance, barely touches nicotinic receptors at all.3PubMed. Anti-nicotinic properties of anticholinergic antiparkinson drugs So when people talk about “anticholinergic side effects,” they are almost always talking about muscarinic blockade.
The Peripheral Symptoms You Notice First
The most immediate anticholinergic effects happen in the body, not the brain, because the glands and smooth muscles that rely on acetylcholine are extremely sensitive to blockade. The salivary glands, bronchial glands, and sweat glands are among the most sensitive targets.4PubMed Central. Anticholinergic medication: Related dry mouth and effects on the salivary glands The result is a predictable set of symptoms that doctors sometimes remember with the mnemonic “dry as a bone, blind as a bat, red as a beet, hot as a hare, mad as a hatter”:
- Dry mouth: Saliva production drops, which can lead to dental problems and difficulty swallowing over time.
- Blurred vision: The muscles that control the pupil and the lens relax, making it hard to focus on nearby objects.
- Constipation: Gut motility slows as the smooth muscles in the intestines lose their acetylcholine signal.
- Urinary retention: The bladder muscle cannot squeeze effectively, making it difficult to empty the bladder.
- Reduced sweating: Sweat glands shut down, which can raise body temperature in warm conditions.
- Rapid heart rate: Acetylcholine normally acts as a brake on heart rate; removing that brake lets the heart speed up.
These peripheral effects can appear even with drugs that are not marketed for any anticholinergic purpose. Many antidepressants, antihistamines, and antipsychotics carry enough anticholinergic activity to trigger dry mouth or constipation as a side effect, even though the drug was designed to do something entirely different.5PubMed Central. Anticholinergic action is rarely a good thing
Effects on the Brain
Because acetylcholine plays a central role in attention, memory, and alertness, blocking it in the brain produces a distinct set of cognitive and behavioral effects. Reviews of the research have found deficits in processing speed, concentration, problem-solving, and language skills, rather than a uniform drop in overall mental ability.6PubMed Central. The cognitive impact of anticholinergics: A clinical review At higher doses or in vulnerable people, the effects can become dramatic: sudden confusion, disorientation, visual hallucinations, and fluctuating consciousness that amounts to delirium.
In one case report, a patient on antipsychotics developed sudden disorientation, behavioral changes, and visual hallucinations. Only after other causes were ruled out was the presentation traced to procyclidine, an anticholinergic drug being used to manage the antipsychotic’s movement-related side effects.7PubMed Central. Delirium secondary to anticholinergics Broader case reviews describe a full spectrum of neuropsychiatric symptoms from anticholinergic agents, including anxiety, agitation, seizures, bizarre behavior, psychosis, and even coma.8PubMed Central. Neuropsychiatric manifestations due to anticholinergic agents and anabolic steroids ingestion: A case series and literature review
Delirium risk rises with what researchers call “anticholinergic burden,” the combined anticholinergic load from every drug a person takes. In a study of hospitalized older patients, those whose total anticholinergic burden scored 3 or higher on a standard scale had roughly three to six times the risk of delirium compared to patients not taking anticholinergic drugs.9PubMed. Relation Between Delirium and Anticholinergic Drug Burden in a Cohort of Hospitalized Older Patients: An Observational Study This is especially insidious because the cumulative effect of several mildly anticholinergic drugs can add up to a substantial total, even when no single drug in the mix would be considered strongly anticholinergic on its own.
Which Drugs Cause Anticholinergic Effects
The list is long and often surprising. A landmark study measuring the anticholinergic activity of 107 commonly used medications in older adults found that the drugs with the strongest activity included some obvious candidates like atropine, but also older tricyclic antidepressants such as amitriptyline and doxepin, antipsychotics like clozapine and thioridazine, the bladder drug tolterodine, and the antispasmodic dicyclomine. A second tier of moderately anticholinergic drugs included diphenhydramine (the active ingredient in many over-the-counter sleep aids and allergy medications), the antidepressants nortriptyline and paroxetine, the antipsychotic olanzapine, and the bladder drug oxybutynin.10PubMed. Anticholinergic activity of 107 medications commonly used by older adults
Even drugs with low anticholinergic activity at standard doses can become relevant. The same study found that medications including citalopram, fluoxetine, quetiapine, ranitidine, and mirtazapine showed measurable anticholinergic activity, and drugs like furosemide, metformin, and hydrocodone showed activity at higher concentrations or in frail patients.10PubMed. Anticholinergic activity of 107 medications commonly used by older adults The clinical significance of these weaker drugs on their own is debatable. Where they become a real problem is in combination: a patient taking a low-dose SSRI, an over-the-counter antihistamine, and a bladder medication may accumulate enough anticholinergic activity to cross a threshold where cognitive side effects appear.
Over-the-counter medications deserve special attention because people take them without medical supervision. Diphenhydramine is probably the biggest culprit. It is the main ingredient in products like Benadryl, Tylenol PM, and many store-brand sleep aids, and it has substantial anticholinergic activity. First-generation antihistamines as a class are significantly more anticholinergic than newer ones like cetirizine or loratadine, which were designed in part to avoid crossing into the brain.
The Dementia Question
The most alarming research on anticholinergic drugs links long-term use to a higher risk of dementia, and the evidence here is more robust than many people realize. A large prospective study following older adults for an average of about seven years found that those with the heaviest cumulative anticholinergic use had about a 54% higher risk of developing dementia and a 63% higher risk of Alzheimer’s disease compared to nonusers, with a clear dose-response pattern.11PubMed Central. Cumulative Use of Strong Anticholinergic Medications and Incident Dementia
A very large case-control study in England involving hundreds of thousands of patients found similar trends. At the highest levels of cumulative exposure, anticholinergic antidepressants, bladder drugs, antiparkinson drugs, and antipsychotics all showed meaningfully elevated dementia odds. Antidepressants with strong anticholinergic properties carried about a 29% increase in dementia risk at high cumulative exposures, while bladder antimuscarinic drugs showed about a 65% increase and antipsychotics about a 70% increase.12JAMA Internal Medicine. Anticholinergic Drug Exposure and the Risk of Dementia A separate case-control study confirmed this and made a particularly striking observation: the association between anticholinergic drug exposure and dementia was visible even when the exposure occurred 15 to 20 years before the diagnosis.13BMJ. Anticholinergic drugs and risk of dementia: case-control study
These are observational studies, so they cannot definitively prove that the drugs cause dementia rather than being taken by people who were already developing it. But the dose-response pattern, the consistency across multiple studies and populations, and the persistence of the association even with exposure windows 15 or more years before diagnosis all strengthen the case for a real biological effect.
How Anticholinergics May Change the Brain
Brain imaging research offers a plausible mechanism. A neuroimaging study of cognitively normal older adults found that those taking anticholinergic medications had greater brain atrophy and reduced brain function in regions that are heavily supplied by acetylcholine neurons, particularly the medial temporal lobe and the cortex.14JAMA Neurology. Association Between Anticholinergic Medication Use and Cognition, Brain Metabolism, and Brain Atrophy in Cognitively Normal Older Adults The researchers proposed that blocking acetylcholine over time may cause actual loss of connections between neurons and even neuronal death in these vulnerable regions. Animal research supports this idea: in mice, damaging the cholinergic neurons in the basal forebrain leads to degeneration of projections to the hippocampus and cortex, two areas critical for memory.
An additional proposed pathway involves stress hormones. Cholinergic neurons help regulate the body’s stress-response system, and reduced cholinergic activity has been linked to higher levels of stress hormones, which in turn are associated with hippocampal cell death and the kinds of protein tangles seen in Alzheimer’s disease.14JAMA Neurology. Association Between Anticholinergic Medication Use and Cognition, Brain Metabolism, and Brain Atrophy in Cognitively Normal Older Adults This is still largely theoretical, but it fits the observational data showing that long-term anticholinergic exposure seems to affect the same brain regions that Alzheimer’s disease targets earliest.
Why Older Adults Are Especially Vulnerable
Older people face a triple threat from anticholinergic drugs. First, aging itself reduces the body’s cholinergic reserves. The brain naturally loses acetylcholine-producing neurons over time, so there is less of a buffer when a drug further blocks what remains. Second, age-related changes in kidney and liver function mean that anticholinergic drugs may linger in the body longer, producing higher blood levels from the same dose. Third, the blood-brain barrier becomes somewhat more permeable with age, allowing more of a drug to reach the brain.15PubMed. The problems of anticholinergic adverse effects in older patients
Compounding all of this, older adults take more medications on average, raising the chance that several mildly anticholinergic drugs will combine into a heavy total burden. A person taking a sleep aid, an antidepressant, and a bladder medication might not think of any one drug as dangerous, but the cumulative anticholinergic load could be substantial.
Measuring Anticholinergic Burden
Because the risk is cumulative, researchers have developed scoring systems to quantify a patient’s total anticholinergic exposure. The two most widely used are the Anticholinergic Drug Scale and the Anticholinergic Cognitive Burden (ACB) scale, both of which assign each medication a score based on its anticholinergic potency and then add up the totals.16PubMed. A review of published anticholinergic scales and measures and their applicability in database analyses Several other tools exist, and clinicians can use them when reviewing a patient’s medication list to flag hidden anticholinergic risk.17PubMed Central. The anticholinergic burden: from research to practice
The challenge with all these scales is that they measure the drug’s inherent anticholinergic potency but cannot perfectly predict what will happen in a specific person. A score of 3 in one patient may cause no noticeable symptoms, while the same score in someone older or on multiple interacting drugs could trigger delirium. The scales are best understood as screening tools, useful for flagging patients who may need a closer look at their medication list rather than definitive risk calculators.
Can Stopping the Drugs Reverse the Damage
This is the question most people want answered, and unfortunately the evidence is mixed. Observational studies have found that cognitive performance tends to improve after anticholinergic medications are discontinued.18PubMed. Impact of anticholinergic discontinuation on cognitive outcomes in older people: a systematic review Individual case reports are encouraging: one published case described a 76-year-old woman who experienced significant cognitive improvement after stopping doxepin, a strongly anticholinergic antidepressant she had been taking long-term.19PubMed. Anticholinergic deprescribing: A case report demonstrating improved cognition and function with minimal adverse withdrawal effects
However, when researchers looked at this question more rigorously through randomized controlled trials, the picture became muddier. A Cochrane systematic review of deprescribing interventions found that the existing trial evidence was of very low certainty and could neither support nor refute the idea that actively reducing anticholinergic medications improves cognitive outcomes in older adults. There was also no clear evidence of improvement in other outcomes like quality of life, physical function, or falls.20PubMed Central. Anticholinergic deprescribing interventions for reducing risk of cognitive decline or dementia in older adults with and without prior cognitive impairment
The gap between what observational studies suggest and what trials have proven likely reflects both the difficulty of running these trials and the possibility that some cognitive damage from years of anticholinergic exposure may not be fully reversible. That does not mean deprescribing is pointless. Even without certainty about cognitive recovery, reducing anticholinergic drugs relieves the peripheral side effects, lowers the risk of delirium, and removes an ongoing insult to the brain’s cholinergic systems. The smart approach, if you suspect your medication list is heavy on anticholinergics, is to talk with your doctor about whether any of those drugs can be swapped for alternatives with less anticholinergic activity. Abrupt discontinuation of some drugs can cause withdrawal effects, so a gradual taper with monitoring is usually the safer route.
Natural and Chemical Sources Beyond Medications
Prescription and over-the-counter drugs account for the vast majority of anticholinergic exposure, but they are not the only sources. Certain plants in the nightshade family produce potent tropane alkaloids, including atropine, hyoscyamine, and scopolamine, that are natural anticholinergic compounds. Deadly nightshade (Atropa belladonna) is the best-known example, and accidental poisonings still occur when its berries are mistaken for edible fruit or when herbal preparations are used carelessly.21PubMed. Toxic Risks of Nightshade Species: A comprehensive review of the documented toxicity of Atropa belladonna, Solanum dulcamara, and Solanum nigrum Jimsonweed (Datura stramonium) is another plant that contains scopolamine and atropine and occasionally causes anticholinergic poisoning, particularly among adolescents who intentionally ingest it for its hallucinogenic effects.
On the chemical side, organophosphate pesticides and nerve agents work by the opposite mechanism: they block the enzyme that breaks down acetylcholine, causing a dangerous flood of acetylcholine activity. The medical treatment for organophosphate poisoning uses anticholinergic drugs (primarily atropine) to counteract that flood.22PubMed Central. Recent advances in the treatment of organophosphorous poisonings In this context, anticholinergic effects are the goal of treatment rather than an unwanted side effect. Atropine and other anticholinergic agents are standard components of military and civilian nerve-agent antidote kits.23PubMed. Organophosphates/nerve agent poisoning: mechanism of action, diagnosis, prophylaxis, and treatment
Acute Anticholinergic Poisoning
Intentional or accidental overdose of anticholinergic drugs can cause a full-blown toxicity syndrome. The classic presentation includes many of the peripheral effects described earlier, cranked up to dangerous levels: a fast heart rate, fever from inability to sweat, flushed dry skin, dilated pupils, and urinary retention. The central effects are where the real danger lies: severe agitation, hallucinations, seizures, and altered consciousness that can progress to coma. Diphenhydramine overdose is one of the more common culprits seen in emergency departments.
The primary antidote is physostigmine, a drug that temporarily blocks the enzyme that breaks down acetylcholine, thereby boosting acetylcholine levels to compete against the offending drug at the receptor. In patients with anticholinergic toxicity, those treated with physostigmine alone had a significantly lower rate of intubation compared to those treated with other approaches, roughly 2% versus 8%.24PubMed Central. The Use of Physostigmine by Toxicologists in Anticholinergic Toxicity In one reported case, physostigmine reversed severe anticholinergic delirium from a massive diphenhydramine overdose and prevented the need for mechanical ventilation entirely.25PubMed Central. Physostigmine in Anticholinergic Poisoning: An Old Antidote With Resurgence
Why the Same Drug Hits Some People Harder
Not everyone taking the same anticholinergic drug at the same dose experiences the same severity of effects, and genetics is part of the reason. Many anticholinergic drugs are broken down in the liver by enzymes called CYP2D6 and CYP2C19. People who carry gene variants that make these enzymes less active, sometimes called “poor metabolizers,” clear the drugs more slowly and end up with higher blood levels. Older adults and older women in particular tend to reach higher concentrations of anticholinergic drugs at standard doses.26PubMed Central. The role of sex, age and genetic polymorphisms of CYP enzymes on the pharmacokinetics of anticholinergic drugs
Recent research has pinpointed specific gene variants that matter. Certain CYP2D6 variants were associated with higher anticholinergic burden scores and worse dry-mouth symptoms at a given dose. In one study, a particular CYP2D6 variant carried roughly a 13-fold relative risk for severe dry mouth compared to people without that variant.27PubMed Central. Pharmacogenetic associations of CYP2D6 and CYP2C19 variants with anticholinergic drug burden and hyposalivation This kind of variation explains why one person on a modest dose of an anticholinergic antidepressant sails through without noticeable side effects while another on the same dose develops intolerable dry mouth, constipation, and brain fog. Pharmacogenomic testing, which identifies your enzyme profile before prescribing, is becoming more common but is still not routine for most anticholinergic drugs.
Sex and body composition matter too. Women generally have higher body fat percentages, and many anticholinergic drugs are fat-soluble, meaning they distribute more widely and can linger longer. Combined with slower enzyme activity that tends to come with aging, this helps explain why older women appear to be disproportionately affected in clinical data.26PubMed Central. The role of sex, age and genetic polymorphisms of CYP enzymes on the pharmacokinetics of anticholinergic drugs