Hyoscyamine

Hyoscyamine is a tropane alkaloid and one of the naturally occurring antimuscarinic compounds found in several plants of the nightshade family, notably Atropa belladonna (deadly nightshade), Datura stramonium (jimsonweed), and Hyoscyamus niger (henbane). It is the levo- or (-)-enantiomer of atropine; atropine is a racemic mixture that contains hyoscyamine and its mirror image.

As a competitive antagonist of muscarinic acetylcholine receptors (M1–M5), hyoscyamine reduces parasympathetic activity. By blocking acetylcholine signaling in smooth muscle and secretory glands, it decreases GI motility and secretions and can affect the heart and central nervous system at higher doses.

Hyoscyamine is used as an antispasmodic to relieve intestinal cramps from conditions such as irritable bowel syndrome and peptic ulcer disease. It also appears in some combination medications for functional GI disorders and has historical use in reducing secretions before surgery.

The drug is available in oral tablets, sublingual tablets, and oral solutions; extended-release formulations exist in some markets. It is usually prescribed on a short-term basis and may be used alone or with other anticholinergic agents.

Common adverse effects include dry mouth, blurred vision, tachycardia, constipation, urinary retention, and confusion, particularly in older patients. Hyoscyamine is contraindicated in narrow-angle glaucoma and certain types of GI obstruction, and should be used with caution in pregnancy and in patients with myasthenia gravis.

Because it competes with other anticholinergic drugs, hyoscyamine can interact with antihistamines, tricyclic antidepressants, and other agents that slow GI motility or affect the central nervous system.