Antimuscarinic effects are side effects that occur when certain medications interfere with the body’s natural chemical messengers. These effects often manifest as a “drying” or “slowing” of various bodily functions. Understanding them is important, as they can impact daily comfort and, in some cases, pose serious health concerns.
The Biological Mechanism
The body’s “rest and digest” functions are largely managed by the parasympathetic nervous system, which relies on acetylcholine (ACh). Acetylcholine transmits signals by binding to muscarinic receptors, found on cells in various organs like glands, smooth muscles, and the heart. When acetylcholine binds, it triggers normal responses such as increased saliva production, muscle contractions in the digestive tract, or a slower heart rate.
Antimuscarinic drugs block these muscarinic receptors, preventing acetylcholine from binding and delivering its message. This blocking action reduces or inhibits typical parasympathetic responses in affected organ systems. For example, blocking muscarinic receptors in salivary glands reduces saliva production, leading to a dry mouth.
Commonly Experienced Symptoms
Antimuscarinic effects can manifest in various parts of the body, leading to noticeable symptoms. Drying effects are frequently reported, such as dry mouth, dry eyes, and reduced sweating. Difficulty swallowing and increased thirst can also accompany a dry mouth.
Visual disturbances are another common set of symptoms. These include blurred vision, often due to the eye’s inability to focus, and increased sensitivity to light caused by dilated pupils. Problems with the digestive and urinary systems are also typical, with individuals often experiencing constipation due to slowed bowel movements and difficulty urinating, known as urinary retention.
The cardiovascular system can also be affected, commonly leading to an increased heart rate, known as tachycardia. Antimuscarinic effects can also extend to the central nervous system, particularly with higher doses or in sensitive individuals. This can result in drowsiness, dizziness, confusion, or memory impairment. More severe central nervous system effects, though less common, can include agitation, disorientation, and hallucinations.
Medications with Antimuscarinic Properties
Many medication categories can possess antimuscarinic properties, either as their primary therapeutic action or as an unintended side effect. First-generation antihistamines, used for allergies and sleep, frequently exhibit these effects; examples include diphenhydramine and chlorpheniramine. Tricyclic antidepressants, such as amitriptyline and imipramine, also have antimuscarinic activity.
Medications for an overactive bladder, like oxybutynin, tolterodine, solifenacin, and darifenacin, work by targeting muscarinic receptors to relax bladder muscles. Certain antipsychotic medications, including olanzapine and clozapine, can also contribute to these effects. Drugs used to manage Parkinson’s disease, such as benztropine and trihexyphenidyl, and some medications for muscle spasms like dicyclomine and hyoscyamine, carry antimuscarinic properties.
Impact on Vulnerable Groups
Certain populations, particularly older adults, are more susceptible to the adverse effects of antimuscarinic medications. This increased sensitivity stems from reduced physiological reserves and changes in how their bodies process medications. Even mild antimuscarinic effects can have a greater impact on their well-being.
“Anticholinergic burden” describes the cumulative effect of taking multiple medications, each contributing to antimuscarinic activity. Older adults frequently take several medications, increasing their overall anticholinergic burden. This burden can lead to an increased risk of falls due to dizziness and blurred vision, and the development of delirium. A higher anticholinergic burden has also been associated with cognitive decline and an increased risk of dementia.