Oxybutynin
Oxybutynin
Generic Name
Oxybutynin
Mechanism
- Anticholinergic (butyrylcholinesterase inhibitors)
- Competitive antagonist of muscarinic M3 receptors on bladder detrusor muscle → ↓ smooth‑muscle contraction.
- Reduces bladder overactivity, urgency, urge incontinence.
- Inhibits secretion of digestive fluids and exerts mild systemic antimuscarinic effects.
> Key point: Oxybutynin’s selectivity is low; thus, it mediates both therapeutic effects and anticholinergic side effects.
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Pharmacokinetics
| Parameter | Details |
| Absorption | Oral bioavailability ~25 %; extended‑release (ER) tablets reduce first‑pass metabolism. |
| Distribution | Highly lipophilic; crosses blood‑brain barrier → CNS side effects. |
| Metabolism | Primarily hepatically → N‑oxide and glucuronide conjugates; minimal CYP involvement. |
| Elimination | Renal clearance ~53 % unchanged; half‑life ~9‑12 h (ER). |
| Drug interactions | ↓ CYP2D6 activity, but interactions limited; avoid concomitant serotonergic meds (SSRIs) to reduce anticholinergic burden. |
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Indications
- Urinary urge incontinence (overactive bladder) – first‑line antimuscarinic.
- Detrusor overactivity for bladder compliance issues.
- Functional gastrointestinal disorders (rarely, constipation or diarrhoea control).
- Pre‑operative colonoscopy prophylaxis (rare off‑label use).
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Contraindications
- Absolute contraindications
- Acute urinary retention (prostate obstruction)
- Intestinal or urinary tract obstruction
- Bowel/bladder Sphincter dysfunction (cystitis)
- Caution in
- Narrow‑angle glaucoma
- Myasthenia gravis, drosten syndrome
- Significant cardiac conduction delay
- Severe hepatic impairment
- Pregnancy Category B (prefer lower‑risk agents)
- Elderly (higher anticholinergic burden)
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Dosing
| Formulation | Starting dose | Titration | Max daily dose | Notes |
| ER tablets | 5 mg PO QHS | Increase by 5 mg every 1–2 weeks based on tolerance | 40 mg/day | Use chew‑free to reduce GI distress. |
| IR (immediate‑release) capsules | 5 mg PO TID | Increase by 5 mg every week | 20 mg/day | Faster onset—use for acute symptoms. |
| Topical gel | 2.5 mg/kg (≤ 200 mg/day) | Titrate to 5 mg/kg | 5 mg/kg** | Avoid ocular contact; reduces systemic side effects. |
| Injections | 4 mg IM | Counsel patient to monitor for post‑op vomiting | Rarely used | Effective for acute bladder spasms or postoperative management. |
*Adjust dose for renal/hepatic impairment; hold in severe dysfunction.*
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Adverse Effects
| Category | Occurrence | Examples |
| Anticholinergic (common) | 35–70 % | Dry mouth, blurred vision, constipation, tachycardia |
| CNS | 10–20 % | Drowsiness, headache, dizziness |
| GI | 5–15 % | Nausea, vomiting, dysphagia |
| Cardiac | < 1 % | Palpitations, QT prolongation, arrhythmia (rare & serious) |
| Other | 1 % | Rash, urticaria |
Serious risks: severe constipation leading to paralytic ileus, acute urinary retention, angioedema, arrhythmias (especially in patients with baseline ECG abnormalities).
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Monitoring
| Parameter | Frequency | Rationale |
| Baseline History | Prior to initiation | Identify contraindications (uveolaryngitis, glaucoma) |
| Renal Function (eGFR) | Every 3–6 mo | Dose adjustment for impaired clearance |
| Liver Enzymes | Every 6–12 mo | High dose may raise transaminates |
| ECG | As indicated (age > 60 yr, cardiac disease) | Detect prolonged QT or conduction delay |
| Weight / BP | Every visit | Fluid retention, postural hypotension |
| Patient diary (urinary urgency, constipation) | At 2 wk, 4 wk, 3 mo | Assess efficacy and tolerance |
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Clinical Pearls
- Chew‑free ER tablets dramatically cut oral dryness and nausea – especially important in elderly patients.
- Topical Oxybutynin offers near‑zero systemic exposure – useful when secondary anticholinergic burden must be kept below a threshold (e.g., polypharmacy in seniors).
- Avoid Oxybutynin in acute urinary retention – may trigger or worsen obstruction; if needed, use a detrusor‑relaxant with fewer antimuscarinic properties.
- Titration schedule: two-week intervals allow proper assessment of both therapeutic benefit and emergence of dry‑mouth intensity; faster titration often leads to poor adherence.
- Drug interactions: combine with cholinesterase inhibitors (e.g., donepezil) may neutralize effect; avoid when possible.
- Pregnancy: Though Category B, avoid if the mother requires both anticholinergic and cholinesterase activity or in the presence of diabetes insipidus.
- Reversible 33‑hour anesthetic blockade: Oxybutynin can be reversed by bethanechol if severe detrusor overactivity persists unacceptably.
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