Anticholinergic drugs are medications that block the action of acetylcholine, a chemical messenger your body uses to control muscles, digestion, heart rate, and many brain functions. They’re prescribed for conditions ranging from overactive bladder to asthma, but dozens of common medications, including over-the-counter allergy pills and sleep aids, also carry anticholinergic effects that many people don’t realize.
How Anticholinergics Work
Acetylcholine is one of the body’s most widespread chemical messengers. It operates across the brain, the nervous system that controls involuntary functions like digestion and heart rate, and the junctions where nerves meet muscles. When a drug blocks acetylcholine from reaching its receptors, the signals it normally sends get dampened or silenced entirely.
Most anticholinergic drugs specifically block what are called muscarinic receptors, the type found in smooth muscle, glands, the heart, and the brain. They generally don’t affect the nicotinic receptors found at the junction between nerves and skeletal muscles. This distinction matters because it explains why these drugs slow your gut and dry your mouth without causing muscle paralysis. Technically, “antimuscarinic” is the more accurate term, but “anticholinergic” is what you’ll see on drug labels and in most medical conversations.
Conditions They Treat
Anticholinergics are used across a surprisingly wide range of medical specialties:
- Overactive bladder and urge incontinence: Drugs like oxybutynin and tolterodine relax the bladder muscle, reducing the urgent need to urinate.
- Respiratory conditions: Inhaled forms like ipratropium and tiotropium open the airways in people with COPD or asthma by relaxing the muscles around the bronchial tubes.
- Parkinson’s disease: Certain anticholinergics help reduce tremor and muscle stiffness by rebalancing chemical signals in the brain.
- Gastrointestinal spasms: Antispasmodic drugs used for cramping and irritable bowel symptoms are often antimuscarinics at their core.
- Allergies: First-generation antihistamines like diphenhydramine carry strong anticholinergic activity, which is partly why they cause drowsiness and dry mouth.
Common Side Effects
Because acetylcholine is involved in so many body processes, blocking it tends to produce a predictable cluster of side effects. These fall into two categories.
Peripheral effects are the ones you feel in your body: dry mouth, constipation, blurred vision, urinary retention, increased heart rate, and reduced sweating. These happen because the drug is suppressing gland secretions and relaxing smooth muscle throughout the body. Dry mouth is often the first side effect people notice, and constipation is the most persistent.
Central effects are the ones that affect your thinking. These include difficulty concentrating, memory problems, confusion, and attention deficits. In older adults especially, these cognitive side effects can be significant enough to look like the early stages of dementia, even though they may be entirely medication-driven.
Medications You Might Not Realize Are Anticholinergic
Many people take anticholinergic drugs without knowing it. Diphenhydramine, the active ingredient in numerous over-the-counter sleep aids and allergy medications, is one of the most widely used anticholinergics in the world. First-generation antihistamines as a group, including chlorpheniramine, doxylamine, hydroxyzine, and meclizine, all carry strong anticholinergic activity.
Several classes of prescription medications also have anticholinergic properties that aren’t obvious from their primary purpose. Certain older antidepressants, some antipsychotics, and various antispasmodics all block muscarinic receptors to varying degrees. Even some common antacids and heart medications have mild anticholinergic effects. This is why the total “anticholinergic burden,” the combined effect of every medication you take, matters more than any single drug on its own.
The Anticholinergic Burden Scale
Healthcare providers use a tool called the Anticholinergic Cognitive Burden (ACB) scale to estimate how much anticholinergic load a person’s medications are creating. Each drug gets a score from 1 to 3. A score of 1 means minimal anticholinergic activity, 2 means moderate, and 3 means strong, definite anticholinergic effects. Your total burden is the sum of all your medications’ individual scores.
Drugs like diphenhydramine and oxybutynin score a 3. Many common antidepressants like sertraline, fluoxetine, and citalopram score a 1, meaning they have low but real anticholinergic activity. Individually, a score of 1 is negligible. But if you’re taking four or five medications that each score a 1 or 2, the cumulative effect can become clinically meaningful, particularly for cognition.
Risks for Older Adults
Anticholinergics pose particular risks for people over 65. The American Geriatrics Society’s Beers Criteria, a widely used guideline for safe prescribing in older adults, recommends avoiding first-generation antihistamines entirely in this age group because of the risk of confusion, constipation, and other anticholinergic toxicity. Cumulative anticholinergic exposure is associated with increased risk of falls and delirium, even in younger adults.
The long-term cognitive risks are concerning. A systematic review and meta-analysis found that anticholinergic use lasting three months or longer increased the risk of dementia by an estimated 46% compared to nonuse. In people with particularly high anticholinergic burden scores, the risk was several times higher. This doesn’t mean everyone who takes an antihistamine will develop dementia, but it does mean that prolonged, heavy use of strongly anticholinergic drugs is a modifiable risk factor worth paying attention to.
Who Should Avoid Them
People with narrow-angle glaucoma are the most well-known group who should not take anticholinergics. These drugs can trigger a sudden, dangerous increase in eye pressure by dilating the pupil and blocking fluid drainage within the eye. Narrow-angle glaucoma has been a standard exclusion criterion in major clinical trials of anticholinergic bladder medications. If you have open-angle glaucoma (the more common type) or narrow-angle glaucoma that has already been surgically treated, anticholinergics are generally considered safe.
Men with an enlarged prostate should also use caution, since anticholinergics can worsen urinary retention. People with severe constipation or a history of bowel obstruction face similar concerns, as these drugs slow gut motility further.
Signs of Anticholinergic Toxicity
When anticholinergic effects become severe, whether from an overdose, drug interactions, or high cumulative burden, the result is a recognizable pattern that medical students learn through an old mnemonic: “dry as a bone” for the absence of sweating, “blind as a bat” for dilated pupils and blurred vision, “red as a beet” for flushed skin, “hot as a hare” for fever caused by the inability to sweat, “mad as a hatter” for delirium, and “full as a flask” for urinary retention.
In practice, the earliest warning signs are usually a noticeable worsening of dry mouth, new constipation, difficulty urinating, or feeling mentally foggy. If you or someone you’re caring for develops sudden confusion, rapid heartbeat, hot and dry skin, or an inability to urinate while taking one or more anticholinergic medications, that combination of symptoms warrants urgent medical attention.
Reducing Your Anticholinergic Burden
If you’re taking multiple medications, ask your pharmacist to review your total anticholinergic burden. Many pharmacies can generate this score quickly. In some cases, switching to a newer medication in the same class with lower anticholinergic activity can preserve the therapeutic benefit while cutting the cognitive side effects. For allergies, second-generation antihistamines like cetirizine and loratadine have far less anticholinergic activity than diphenhydramine. For sleep, non-anticholinergic options exist that don’t carry the same cognitive risks.
The goal isn’t necessarily to eliminate every anticholinergic medication. Some conditions genuinely require them, and the benefits can outweigh the risks. The goal is to make sure you’re not accumulating anticholinergic burden from medications you could easily swap, especially if you’re over 65 or already noticing memory or concentration changes.