Hyoscyamine

Hyoscyamine is a competitive antagonist at muscarinic receptors and is roughly twice as potent as racemic atropine, since only the L-isomer is active. It is used for the cramping of irritable bowel syndrome and diverticulitis, for bladder spasm, as a drying agent to reduce secretions in palliative care, and as an adjunct in peptic ulcer disease, though proton pump inhibitors have superseded that role. Its adverse effects are those of muscarinic blockade: xerostomia, mydriasis with cycloplegia and blurred vision, tachycardia, constipation, urinary retention, anhidrosis with hyperthermia, and central confusion, agitation, and hallucinations, the last being more prominent because it is a tertiary amine that crosses the blood-brain barrier — unlike quaternary agents such as glycopyrrolate and ipratropium, which do not. It should be distinguished from hyoscine (scopolamine), used transdermally for motion sickness. Contraindications mirror those of dicyclomine, and the drug should be avoided in the elderly because of anticholinergic cognitive burden.