OXYBUTYNIN
Clinical safety rating: safe
Animal studies have demonstrated safety
Oxybutynin is an anticholinergic agent that competitively antagonizes muscarinic acetylcholine receptors (M1, M2, M3 subtypes) in the bladder detrusor muscle, inhibiting involuntary contractions and increasing bladder capacity.
| Metabolism | Extensive hepatic metabolism via cytochrome P450 enzymes, primarily CYP3A4, to active metabolite N-desethyloxybutynin. Also involves CYP2C9 and CYP2C19. |
| Excretion | Primarily hepatic metabolism; less than 1% excreted unchanged in urine. Metabolites are mainly excreted renally (50%) and fecally (40%). |
| Half-life | Terminal half-life: 12-13 hours (range 7-20 hours) in healthy adults. In elderly, half-life may be prolonged due to reduced clearance. |
| Protein binding | Approximately 91-93% bound, primarily to alpha-1-acid glycoprotein and albumin. |
| Volume of Distribution | 1.6-3.0 L/kg (100-200 L), indicating extensive tissue distribution. |
| Bioavailability | Oral immediate-release: ~6% (extensive first-pass metabolism); extended-release: approximately 15-20% (improved relative to IR); transdermal: ~10-15% (avoiding first-pass); topical gel: ~5-10%. |
| Onset of Action | Oral immediate-release: 30-60 minutes; oral extended-release: 3-6 hours; transdermal: 24-48 hours; topical gel: within 2 hours. |
| Duration of Action | Oral immediate-release: 6-10 hours (requires dosing 2-3 times daily); oral extended-release: 24 hours (once-daily dosing); transdermal: 96-hour wear period; topical gel: 24 hours. |
5 mg orally 2-3 times daily; maximum 5 mg 4 times daily. Extended-release: 5-10 mg orally once daily; maximum 30 mg/day. Transdermal: 3.9 mg/day patch applied every 3-4 days. Topical gel: 1 g (3 pumps) applied once daily.
| Dosage form | FILM, EXTENDED RELEASE |
| Renal impairment | CrCl 15-29 mL/min: reduce dose or increase interval; avoid extended-release if CrCl <30 mL/min. CrCl <15 mL/min: not recommended (no data). |
| Liver impairment | Child-Pugh Class C: avoid use. Child-Pugh Class A or B: no specific dose adjustment recommended, but use with caution. |
| Pediatric use | Children ≥5 years: immediate-release 5 mg orally 2-3 times daily; maximum 5 mg 4 times daily. Extended-release: not recommended in children <18 years. Transdermal: not recommended for pediatric use. |
| Geriatric use | Start at 2.5 mg orally 2-3 times daily for immediate-release; increase gradually. Extended-release: 5 mg orally once daily; maximum 10 mg/day. Transdermal: 3.9 mg/day patch every 3-4 days. Monitor for anticholinergic effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Other anticholinergic drugs can have additive effects Can cause blurred vision and urinary retention.
| Breastfeeding | Oxybutynin is excreted into breast milk in small amounts. The estimated infant dose is <0.1% of maternal weight-adjusted dose. M/P ratio: not established. No adverse effects reported in breastfed infants. Use with caution, especially in preterm or neonates due to anticholinergic sensitivity. |
| Teratogenic Risk | Oxybutynin is classified as FDA Pregnancy Category B. Animal studies have not demonstrated fetal risk, but no adequate and well-controlled studies in pregnant women exist. First trimester: No known teratogenic effects; risk cannot be excluded. Second and third trimesters: Potential for anticholinergic effects (e.g., decreased fetal heart rate variability) at high doses; no malformation risk identified. |
■ FDA Black Box Warning
None
| Common Effects | Dry mouth |
| Serious Effects |
["Urinary retention","Gastric retention","Uncontrolled narrow-angle glaucoma","Hypersensitivity to oxybutynin or any component","Concurrent use with other anticholinergics may be contraindicated in certain conditions"]
| Precautions | ["May aggravate conditions worsened by anticholinergic effects (e.g., myasthenia gravis, ulcerative colitis, intestinal atony, hiatal hernia, prostatic hyperplasia, urinary retention)","Risk of heat prostration in hot environments due to decreased sweating","Angioedema reported (discontinue if occurs)","CNS effects (e.g., somnolence, dizziness, confusion) especially in elderly, may impair ability to drive or operate machinery","Hepatic impairment: use with caution, dose adjustment may be needed","Renal impairment: dose adjustment recommended for CrCl <30 mL/min"] |
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| Fetal Monitoring | Monitor maternal heart rate, blood pressure, and urinary retention. Fetal assessment: Consider nonstress test or biophysical profile if maternal anticholinergic toxicity suspected (high doses). No routine fetal monitoring required. |
| Fertility Effects | No known effects on fertility in animal studies. In humans, no documented evidence of impaired fertility. Anticholinergic effects may theoretically impact vaginal lubrication, but no established association. |