Hyoscyamine (also known as daturine or duboisine) is a naturally occurring tropane alkaloid and plant toxin. It is a secondary metabolite found in certain plants of the family Solanaceae, including henbane, mandrake, angel's trumpets, jimsonweed, the sorcerers' tree, and Atropa belladonna (deadly nightshade). It is the levorotary isomer of atropine (third of the three major nightshade alkaloids) and thus sometimes known as levo-atropine.^{[better source needed]}

In 2021, it was the 272nd most commonly prescribed medication in the United States, with more than 900,000 prescriptions.

Brand names for hyoscyamine include Symax, HyoMax, Anaspaz, Egazil, Buwecon, Cystospaz, Levsin, Levbid, Levsinex, Donnamar, NuLev, Spacol T/S, and Neoquess.

Hyoscyamine is used to provide symptomatic relief of spasms caused by various lower abdominal and bladder disorders including peptic ulcers, irritable bowel syndrome, diverticulitis, pancreatitis, colic, and interstitial cystitis. It has also been used to relieve some heart problems, control some of the symptoms of Parkinson's disease, as well as for control of abnormal respiratory symptoms and "hyper-mucus secretions" in patients with lung disease.

It is also useful in pain control for neuropathic pain, chronic pain and palliative care — "comfort care" — for those with intractable pain from treatment resistant, untreatable, and incurable diseases. When combined with opioids it increases the level of analgesia (pain relief) obtained. Several mechanisms are thought to contribute to this effect. The closely related drugs atropine and hyoscine and other members of the anticholinergic drug group like cyclobenzaprine, trihexyphenidyl, and orphenadrine are also used for this purpose. When hyoscyamine is used along with opioids or other anti-peristaltic agents, measures to prevent constipation are especially important given the risk of paralytic ileus.

Side effects include dry mouth and throat, increased appetite leading to weight gain, eye pain, blurred vision, restlessness, dizziness, arrhythmia, flushing, and faintness. An overdose will cause headache, nausea, vomiting, and central nervous system symptoms including disorientation, hallucinations, euphoria, sexual arousal, short-term memory loss, and possible coma in extreme cases. The euphoric and sexual effects are stronger than those of atropine but weaker than those of hyoscine, as well as dicycloverine, orphenadrine, cyclobenzaprine, trihexyphenidyl, and ethanolamine antihistamines like phenyltoloxamine.

Hyoscyamine is an antimuscarinic; i.e., an antagonist of muscarinic acetylcholine receptors. It blocks the action of acetylcholine at sweat glands (sympathetic) and at parasympathetic sites in salivary glands, stomach secretions, heart muscle, sinoatrial node, smooth muscle in the gastrointestinal tract, and the central nervous system. It increases cardiac output and heart rate, lowers blood pressure and dries secretions. It may antagonize serotonin.^{[better source needed]} At comparable doses, hyoscyamine has 98 percent of the anticholinergic power of atropine; the other major Atropa belladonna-derived drug hyoscine (known in the United States as scopolamine) has 92 per cent of the antimuscarinic potency of atropine.^{[better source needed]}

Hyoscyamine has been described as a selective muscarinic acetylcholine M₂ receptor antagonist without significant effects at the other muscarinic acetylcholine receptors. This is in contrast to related antimuscarinics like atropine and scopolamine, which are non-selective antagonists of all five muscarinic acetylcholine receptors. Antagonism of both the muscarinic acetylcholine M₁ receptor and M₂ receptor has been implicated as having negative effects on memory and cognition. Hyoscyamine has been described as having deliriant effects similarly to scopolamine, atropine, and other antimuscarinics. However, other sources have reported that hyoscyamine potently antagonizes all five muscarinic acetylcholine receptor subtypes.