Act directly on muscarinic cholinergic receptors, binding reversibly and causing activation.  

Muscarinic activation effects the heart, exocrine glands, smooth muscles, and eye.  Activation of the muscarinic receptors on these organs mimics the parasympathetic effects (hence the term parasympathomimetic agents) so the outcomes are:  bradycardia; increased secretion of sweat, saliva and gastric acid as well as increased bronchial secretions; contraction of smooth muscles of the GI tract, bladder and bronchi; relaxation of smooth muscle in vasculature; miosis and ciliary muscle contraction.  Bethanechol is used to treat urinary retention.

Side effects are rare with oral administration and higher with injection.  Can cause hypotension and bradycardia; excessive salivation, increased secretion of gastic acid, abdominal cramps and diarrhea; bronchoconstriction; dysrhythmias.

This drug is contraindicated in patients with gastric ulcers, intestinal obstruction, urinary tract obstruction or weak bladder wall, asthma or hyperthyroidism.

Competitive blockade at muscarinic receptors.  All responses are caused by prevention of receptor activation by acetylcholine.

Primarily influence the heart, exocrine glands, smooth muscles and eyes.  Increases heart rate; decreases secretion from salivary, bronchial and sweat glands and from the acid-secreting cells of the stomach; causes relaxation of smooth muscle of the bronchi, urinary bladder, and GI tract; mydriasis and cycloplegia of the eyes; can cause CNS stimulation.  Remember, there is no innervation of the vasculature by parasympathetic fibers, so these drugs have no effect on blood vessels.  Another important thing to remember about these drugs is that the effects produced at muscarinic receptors of different structures is based on receptor sensitivity.  It takes high doses to effect the stomach and lungs for example, and by giving doses that high all other structures with muscarinic receptors will be affected.

The therapeutic uses of muscarinic antagonists include: preanesthetic medication; use during ophthalmic exams and eye surgery; treatment of bradycardia, biliary colic, intestinal hypertonicity and hypermotility (as would be caused in disorders like dysentery and diverticulitis); as an antidote to muscarinic agonist poisons; as a last resort treatment for peptic ulcer disease.

Dry mouth, blurred vision and photophobia, elevation of intraocular pressure, urinary retention, constipation, anhidrosis, tachycardia, exacerbation of asthma.

Antihistamines, phenothiazine antipsychotics, and tricyclic antidepressants all have muscarinic receptor blocking capabilities, and would potentiate the effects of muscarinic antagonists.

Contraindicated for patients with glaucoma, tachycardia, urinary tract obstruction and intestinal atony, and should be used cautiously in patients with asthma.  Teach patient how to relieve xerostomia, and to avoid vigorous exercise. Warn patient to be cautious due to impaired  vision and photophobia.  Advise patient to void before taking medication to avoid urinary retention, and to increase fluids and fiber to reduce constipation.