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Hyoscyamine is the levorotatory (S) enantiomer of atropine and a tropane alkaloid found in belladonna, henbane, jimsonweed, and related nightshades; atropine itself is the racemate, and the l-hyoscyamine form carries essentially all of the mixture's antimuscarinic potency. It acts as a competitive, non-selective antagonist at muscarinic acetylcholine receptors, and is used medically as a gastrointestinal and genitourinary antispasmodic to relieve smooth-muscle cramping. As a classical anticholinergic it dose-dependently reduces secretions and raises heart rate, and its systemic absorption even from ophthalmic use can produce peripheral antimuscarinic effects. At high doses it can precipitate the full anticholinergic toxidrome, with agitation, delirium, and vivid hallucinations, a state reversible with the cholinesterase inhibitor physostigmine and responsible for the deliriant reputation of the nightshade alkaloids.
- Relief of gastrointestinal cramping and spasm
- Treatment of irritable bowel and functional gut disorders
- Reduction of bladder spasm
- Drying of secretions in some settings
- Levorotatory active enantiomer of atropine
- Dry mouth and blurred vision
- Constipation and urinary retention
- Rapid heartbeat and flushing
- Confusion, hallucinations, and delirium at high doses
Mechanism
Hyoscyamine is a competitive at receptors (M1-M5). Because atropine is the racemic mixture and hyoscyamine is its active left-handed half, hyoscyamine carries most of atropine's muscarinic-blocking punch. Peripherally it relaxes the smooth muscle of the gut and urinary tract and reduces secretions, which is the basis of its antispasmodic use; it also crosses into the brain, so high doses block central muscarinic receptors and cause delirium.
receptor fingerprint
M3 receptor (smooth muscle/glands)Antagonist
receptors (M1-M5)Antagonist
Safetyrisks and cautions, not medical advice
At therapeutic doses hyoscyamine mainly causes dry mouth, blurred vision, constipation, and difficulty urinating. Push the dose and it produces the full anticholinergic toxidrome (blurred vision, delirium, flushing, dangerous overheating, dry mouth), plus racing heart, urinary retention, seizures, and death; as a deliriant it brings realistic hallucinations mistaken for reality, confusion, and memory loss, and the experience is dangerous and usually deeply unpleasant. Never combine it with other anticholinergics or antihistamines, since the anticholinergic burden stacks, and remember elderly people and long-term use carry dementia-risk associations. Not medical advice.
Interactionsdocumented pairs only, not exhaustive
Hyoscyamine is a muscarinic antagonist, and nearly all of its interactions are additive antimuscarinic load. Combined with antihistamines such as diphenhydramine, tricyclic antidepressants, low-potency antipsychotics, amantadine, antiparkinsonian agents or other antispasmodics, the result is dry mouth, blurred vision, urinary retention, constipation and, in older adults, confusion and frank delirium. Anticholinergics also impair sweating, so heat intolerance and heatstroke risk rise in hot conditions.
Because hyoscyamine slows gastric emptying and gut transit, it changes how other oral drugs are absorbed. It directly opposes prokinetics such as metoclopramide, and it prolongs contact between solid potassium chloride preparations and the gut wall, which has been linked to mucosal ulceration. Antacids and adsorbent antidiarrhoeals reduce hyoscyamine's own absorption, so simultaneous dosing is usually avoided.
Hyoscyamine also antagonises cholinesterase inhibitors used in myasthenia gravis and in dementia, blunting the benefit of both.
Checking a whole stack? Run it through interactions + stacks.
Subjective profileweighing the evidence above
Reasonable short-term relief for gut or bladder cramping when a clinician prescribes it, and not much more; the anticholinergic load is the whole cost, and dry mouth, constipation and blurred vision arrive with the benefit. Long-term anticholinergic use in older adults carries dementia-risk associations worth taking seriously.
Resources
This entry is here for reference.
Research
- 1965first citedSOME PHARMACOLOGICAL STUDIES ON THE OPTICALLY ACTIVE ISOMERS OF HYOSCINE AND HYOSCYAMINE
- 2021most recentAntispasmodics for Chronic Abdominal Pain: Analysis of North American Treatment Options
- 1.Antispasmodics for Chronic Abdominal Pain: Analysis of North American Treatment Options
- 2.SOME PHARMACOLOGICAL STUDIES ON THE OPTICALLY ACTIVE ISOMERS OF HYOSCINE AND HYOSCYAMINE
- 3.Common anticholinergic solanaceaous plants of temperate Europe - A review of intoxications from the literature (1966-2018).
- 4.Systemic bioavailability of ocularly applied 1% atropine eyedrops.
- 5.Pharmacokinetics and pharmacodynamics in clinical use of scopolamine.
- 6.Physostigmine Reversal of Dysarthria and Delirium After Iatrogenic Atropine Overdose From a Dental Procedure.
- 7.Treatment of irritable bowel syndrome.
- 8.The pharmacology of medieval sedatives: the "Great Rest" of the Antidotarium Nicolai.
- 9.A case of pediatric age anticholinergic intoxication due to accidental Datura stramonium ingestion admitting with visual hallucination.
9 listed here; entry last updated August 2026
Reviews
My notesprivate to this device
FAQ
How is hyoscyamine different from atropine?
Atropine is the racemic (fifty-fifty) mixture of the right- and left-handed molecules, while hyoscyamine is the active left-handed isomer alone. They act on the same muscarinic receptors, and hyoscyamine carries most of atropine's activity.
What is it prescribed for?
Most often as a gastrointestinal antispasmodic, to calm cramping and spasm in conditions like irritable bowel syndrome, and sometimes for bladder spasm.
Can it cause a deliriant high?
Yes, at doses well above the therapeutic range it blocks central muscarinic receptors and causes an anticholinergic delirium. That state is dangerous and typically frightening, not enjoyable.
Adverse effects
- Dry mouth and blurred vision
- Constipation and urinary retention
- Rapid heartbeat and flushing
- Confusion, hallucinations, and delirium at high doses