Hyoscyamine is a chemical compound, a tropane alkaloid it is the levo-isomer to atropine. It is part of the Secondary metabolites of plants.

It is used in medicine under a number of brand names but also as a generic compund. It is used to provide symptomatic relief to various gastrointestinal disorders including spasms, peptic ulcers, IBS, pancreatitis, colic and cystitis. It has also been used to relieve some heart problems and control some of the symptoms of Parkinson's disease. It is anticholinergic, working by inhibting the action of acetylcholine in smooth and cardiac muscle, the sinoatrial and atrioventricular nodes and the exocrine glands.

Side effects include eye pain, blurred vision, restlessness, dizziness, arrythmia, flushing, faintness. An overdose will cause headache, nausea, vomiting and CNS symptoms including disorientation, hallucinations, euphoria, inappropriate affect, short-term memory loss and coma.

It can be extracted from plants of the Solanaceae family, notably Datura stramonium. Empirically it is C₁₇H₂₃NO₃. Its structural name is α-(hydroxymethyl)-, 8-methyl-8-azabicyclo[3.2.1]oct-3-yl ester, [3(S)-endo]-1αH,5αH-Tropan-3α-ol. It is also known as benzeneacetic acid.