Generic Name

Hyoscyamine

Mechanism

* M₁–M₃ receptor blockade – competitively inhibits acetylcholine at muscarinic cholinergic receptors on smooth muscle and exocrine glands.
* Antimuscarinic effect – reduces involuntary spasms, secretions, and GI motility; decreases urinary urgency and colonic gas.
* No β‑adrenergic or α‑adrenergic activity – limits cardiovascular side‑effects compared with some cardioactive agents.

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Pharmacokinetics

* Absorption – Oral bioavailability ≈ 15–20 %; high first‑pass metabolism in the liver.
* Distribution – Low protein binding; limited lipid solubility (quaternary ammonium).
* Metabolism – Predominantly hepatic glucuronidation; minor CYP involvement (CYP3A4).
* Elimination – Urinary excretion of unchanged drug and metabolites.
* Half‑life – 2–4 h (short, enables multiple daily dosing).

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Indications

* Functional gastrointestinal disorders – irritable bowel syndrome, dyspepsia, colicky abdominal pain.
* Hyperactive bladder/urinary urgency – reduces detrusor overactivity.
* Anxiety‑related gastrointestinal symptoms – e.g., “nervous stomach.”
* Pre‑operative antispasmodic in endoscopy – reduces intestinal motility during procedures.

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Contraindications

* History of acute angle‑closure glaucoma – contraindicated.
* Severe constipation or ileus – avoid due to anti‑secretion and motility suppression.
* Myasthenia gravis – worsens muscle weakness.
* Pregnancy category C – limited data; use only if benefits outweigh risks.
* Kidney impairment – adjust dose; increased urinary retention risk.

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Dosing

* Titration – start at the lowest dose, increase every 3–5 days if needed.

* Administration – take with food to reduce nausea.

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Adverse Effects

Common:
• Dry mouth
• Dysphagia or throat dryness
• Blurred vision (diplopia, mydriasis)
• Constipation
• Dizziness

Serious:
• Urinary retention (especially in men with BPH)
• Paradoxical CNS excitation (especially in the elderly)
• Exacerbation of cardiac conduction abnormalities (rare)
• Severe GI dysmotility leading to ileus

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Monitoring

* Baseline: vision, urinary flow, bowel habits, cardiac rhythm (for patients with ECG changes).

* During therapy:
• Renal function tests if chronic use or renal impairment.
• Monitor for signs of ocular dilation or angle‑closure.
• Watch for constipation or ileus; adjust dose or add stool softener if needed.

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Clinical Pearls

1. Quaternary Ammonium Advantage – Because hyoscyamine is charged, it does not readily cross the blood–brain barrier; CNS side‑effects are lower than tertiary amines (e.g., atropine).

2. Rectal Route – Ideal for patients with vomiting or inability to swallow; delivers effective plasma levels with minimal first‑pass metabolism.

3. Combination Therapy – Pair with laxatives (e.g., polyethylene glycol) in patients prone to constipation to achieve smoother bowel pattern.

4. Avoid in BPH Care – Patients with benign prostatic hyperplasia should be switched to alpha‑blockers to prevent urinary retention.

5. Reversible Binding – Hyoscyamine’s antimuscarinic effect is reversible; withdrawal is usually sudden, so gradual tapering helps mitigate rebound GI spasms.

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• *This drug card is for educational purposes only and does not replace professional medical advice.*