The four most commonly used anticholinergic drugs are diphenhydramine (Benadryl), oxybutynin (Ditropan), amitriptyline (Elavil), and tolterodine (Detrol). Each one is prescribed for a different condition, but they all work the same basic way: blocking a chemical messenger called acetylcholine that controls functions like digestion, urination, and saliva production. That shared mechanism is also why they share a similar set of side effects, some of which can be serious over time.
How Anticholinergic Drugs Work
Acetylcholine is one of the main signals your parasympathetic nervous system uses to manage “rest and digest” functions. It tells your bladder to contract, your mouth to produce saliva, your gut to move food along, and your eyes to focus up close. Anticholinergic drugs block that signal, which dials down activity in whichever organ system a doctor is trying to calm. The trade-off is that the blocking isn’t precise. You can’t shut down acetylcholine in just the bladder without also affecting the eyes, mouth, and brain, which is why side effects show up across multiple body systems at once.
Diphenhydramine (Benadryl)
Diphenhydramine is the most widely recognized anticholinergic because it’s available over the counter and used by millions of people for allergies and sleep. It’s also the active ingredient in many nighttime cold medicines and OTC sleep aids. The standard allergy dose is 25 to 50 mg every six to eight hours, while the sleep dose is typically 50 mg taken 30 minutes before bed.
What most people don’t realize is that diphenhydramine has moderate to high anticholinergic potency. The drowsiness that makes it useful for sleep is actually a central nervous system side effect. Along with sedation, it commonly causes dry mouth, blurred vision, constipation, and urinary retention. The 2023 American Geriatrics Society Beers Criteria lists all first-generation antihistamines, including diphenhydramine, as potentially inappropriate for older adults because tolerance to the sedation develops quickly, but the anticholinergic effects persist.
Oxybutynin (Ditropan)
Oxybutynin is one of the most prescribed medications for overactive bladder. It works by relaxing the bladder muscle, reducing sudden urges to urinate and cutting down on episodes of incontinence. Improvement can start within the first two weeks, but the full benefit often takes six to eight weeks to appear.
Because oxybutynin is strongly anticholinergic, its side effect profile is predictable: dry mouth (often the most bothersome), dry eyes, dry skin, blurred vision, constipation, dizziness, and confusion. Some people also experience stomach pain, headache, flushing, or difficulty sleeping. More serious reactions like a fast or irregular heartbeat, difficulty breathing, or swelling of the face and throat are rare but require immediate medical attention. For people who find the side effects intolerable, newer bladder medications that work through a different mechanism can relax the bladder muscle without the same anticholinergic burden. One such alternative, mirabegron (Myrbetriq), avoids the dry mouth and cognitive effects, though it can raise blood pressure.
Amitriptyline (Elavil)
Amitriptyline is one of the older tricyclic antidepressants, and it carries one of the highest anticholinergic loads of any commonly prescribed medication. On the Anticholinergic Cognitive Burden Scale, it scores a 3 out of 3, meaning there’s evidence it can cause delirium. Today it’s prescribed less often for depression (newer antidepressants have fewer side effects) and more often for chronic pain conditions, nerve pain, migraines, and insomnia at lower doses.
Even at those lower doses, the anticholinergic effects are significant. Dry mouth, constipation, blurred vision, urinary retention, drowsiness, and weight gain are common. The Beers Criteria flags amitriptyline specifically because its clearance slows with age, meaning the drug stays in the body longer in older adults. That raises the risk of confusion, falls, and other complications. Several other antidepressants in the same class, including imipramine, nortriptyline, and doxepin at higher doses, carry similar warnings.
Tolterodine (Detrol)
Tolterodine is the other major anticholinergic bladder medication. Like oxybutynin, it blocks acetylcholine at the bladder muscle to reduce urgency and frequency. It was developed specifically to be more bladder-selective than oxybutynin, meaning it was designed to cause less dry mouth and fewer cognitive side effects. In practice, many people do tolerate it somewhat better, though dry mouth and constipation are still common. The extended-release version tends to produce fewer side effects than the immediate-release form.
Tolterodine still appears on the Beers Criteria list of anticholinergic drugs to avoid in older adults, alongside the other bladder antimuscarinics like solifenacin, darifenacin, and fesoterodine. For older adults already taking other medications with anticholinergic properties, adding any of these bladder drugs can push the total anticholinergic load into a range associated with real cognitive risk.
Why Anticholinergic Burden Matters
The concern with anticholinergic drugs isn’t just about one medication. It’s about the total load across everything you take. A person using diphenhydramine for sleep, oxybutynin for bladder control, and a muscle relaxant for back pain could be stacking three strongly anticholinergic drugs without realizing it. Cumulative exposure to anticholinergic drugs is associated with an increased risk of falls, delirium, and dementia, and this risk applies even in younger adults, not just the elderly.
Clinicians use tools like the Anticholinergic Cognitive Burden Scale to add up the scores of all medications a patient takes. Each drug gets a score from 1 (mildly anticholinergic) to 3 (strongly anticholinergic, with evidence it can cause delirium). The total score gives a picture of overall risk. Drugs scoring a 3 include amitriptyline, oxybutynin, diphenhydramine, and several antipsychotics and antispasmodics. Using more than one medication with anticholinergic properties significantly increases the chance of cognitive decline, delirium, and falls or fractures.
Common Side Effects Across All Four
Because these drugs all block the same chemical messenger, the side effects overlap heavily:
- Dry mouth is the most frequently reported complaint and can affect eating, speaking, and dental health over time.
- Constipation results from slowed gut movement and can become chronic with daily use.
- Blurred vision happens because the muscles that focus your eyes rely on acetylcholine.
- Urinary retention is paradoxically common even with bladder drugs, since the medication can relax the bladder too much in some people.
- Drowsiness and confusion are central nervous system effects, more pronounced with diphenhydramine and amitriptyline.
- Dizziness increases fall risk, particularly in older adults or anyone taking multiple sedating medications.
These effects tend to be dose-dependent, meaning they get worse at higher doses and when multiple anticholinergic drugs are combined. For many people, switching to a non-anticholinergic alternative or reducing the total number of anticholinergic medications can meaningfully improve quality of life, especially cognitive sharpness and energy levels.