Anticholinergics · What Are They, How Do They Work, and More

The prefix “anti” means “against,” while “cholinergics” means “relating to the effects of acetylcholine.” Therefore, anticholinergics work by inhibiting the action of acetylcholine in the body.

The difference between anticholinergics and cholinergics is that anticholinergics work by inhibiting cholinergic receptors and diminishing the action of acetylcholine, while cholinergics work by activating cholinergic receptors and enhancing the action of acetylcholine. 

Anticholinergic drugs refer to agents that block cholinergic receptors, or acetylcholine receptors, and are divided into two categories: antimuscarinics, which block muscarinic receptors (i.e., acetylcholine receptors sensitive to muscarine that function in the central and peripheral nervous system and serve as an interface between the peripheral nervous system and various organs), and antinicotinics, which block nicotinic receptors (i.e., acetylcholine receptors sensitive to nicotine that function in the central nervous system and the neuromuscular junction). Antimuscarinics (e.g., atropine, scopolamine) are used for a variety of conditions, including overactive bladder syndrome, a lower urinary tract condition more common in older people that causes strong urges to urinate and urine leakage; irritable bowel syndrome, an intestinal condition that causes abdominal discomfort, gas, diarrhea, and constipation; and bradycardia, or slower than usual heart rate. Antinicotinics (e.g., bupropion, ipratropium, atracurium, and cisatracurium) are used to support smoking cessation; for asthma or COPD maintenance and treatment; for muscle relaxation during anesthesia; as well as for the treatment of depression in combination with other medications.   

Anticholinergics work by competitively inhibiting the binding of acetylcholine to muscarinic and nicotinic receptors. As a result, they block the action of acetylcholine, which is the chief chemical messenger controlling functions of the parasympathetic nervous system. Parasympathetic functions are typically involuntary processes to conserve energy and include contraction of smooth muscles (e.g., in the bronchioles of the lungs causing bronchospasm), dilation of blood vessels, increased bodily secretions, increased gastrointestinal activity, and decreased heart rate. Blocking the action of acetylcholine can ultimately decrease exocrine gland secretion, increase heart rate, decrease motility of smooth muscles that are typically involved in involuntary muscle movements (e.g., bronchodilation, bladder and bowel movements), or influence vision and neurologic function. 

Anticholinergic effects are the side effects of anticholinergic medications. Effects can include dry mouth and sore throat, resulting from the impaired secretion by exocrine glands. Another side effect is tachycardia, or a higher than usual heart rate, that occurs when anticholinergics interfere with receptors that typically slow the heart rate. Anticholinergics can also cause urinary retention (i.e., the inability to empty the bladder) and obstipation (i.e., the inability to pass stools or gas) due to decreased smooth muscle motility and tone. Additional anticholinergic effects include blurred vision and light sensitivity. Rarely, anticholinergics can cross the blood brain barrier and enter the central nervous systems, causing mood changes, hallucinations, confusion, or disorientation.

Anticholinergic toxicity occurs when anticholinergic agents accumulate in the body and may lead to anticholinergic syndrome, which can cause dry mouth, flushed skin, hyperthermia, and tachycardia. It may also lead to anticholinergic delirium, which is characterized by confusion, hallucinations, and psychomotor symptoms. With lower doses, acute anticholinergic syndrome can resolve on its own after the anticholinergic medication has been stopped and fully excreted. On the other hand, higher doses can be life-threatening, so individuals should be medically reviewed and acetylcholinesterase inhibitors, such as donepezil and rivastigmine, can be prescribed as an antidote.  

Anticholinergics block the action of acetylcholine and can be classified into two categories: antimuscarinics, which block muscarinic receptors, and antinicotinics, which block nicotinic receptors. Antimuscarinics  can be used for overactive bladder syndrome, irritable bowel syndrome, and bradycardia. On the other hand, antinicotinics can be used for smoking dependence, muscle relaxation in anesthesia, and depression. Blocking the action of acetylcholine can cause dry mouth, sore throat, tachycardia, urinary retention, obstipation, as well as visual and neurologic disturbances. Anticholinergic toxicity is caused by increased accumulation of anticholinergic agents in the body and can lead to anticholinergic syndrome. In higher doses, anticholinergic syndrome can be life threatening and requires immediate medical advice and treatment with acetylcholinesterase inhibitors.