Well they're also an anticholinergic, which means they block the neurotransmitter acetylcholine. Acetylcholine is the chief chemical messenger controlling functions of the parasympathetic nervous system. It's really important in brain and muscle function.

What are the effects of anticholinergics? 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.

What is anticholinergic toxicity? 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.

Fyi. Anticholinergics are a class of drugs which have been linked to dementia when used regularly by those 65 and older. Imbalances in acetylcholine are linked with chronic conditions, such as Alzheimer's disease and Parkinson's disease.