PHENETRON
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PHENETRON (PHENETRON).
Phenetron is an antihistamine that competes with histamine for H1-receptor sites, blocking histamine-mediated effects in the respiratory tract, vascular system, and gastrointestinal tract. It also exhibits anticholinergic and sedative properties.
| Metabolism | Hepatic via CYP450 isoenzymes, primarily CYP2D6; metabolites undergo renal excretion. |
| Excretion | Renal: ~70% unchanged; Biliary/Fecal: ~15% as metabolites; 15% unidentified |
| Half-life | Terminal half-life 12–15 hours; clinically, steady-state achieved in ~3 days |
| Protein binding | 98% bound to albumin |
| Volume of Distribution | 1.5–2.0 L/kg; indicates extensive tissue distribution |
| Bioavailability | Oral: 40–60% due to first-pass metabolism; IM: 75–85%; IV: 100% |
| Onset of Action | Oral: 30–60 minutes; IV: 2–5 minutes; IM: 10–15 minutes |
| Duration of Action | Oral: 6–8 hours; IV: 4–6 hours; IM: 4–6 hours; sustained-release formulations: 12–24 hours |
Adults: 50 mg intramuscularly every 6 hours as needed.
| Dosage form | TABLET |
| Renal impairment | GFR <30 mL/min: 25 mg every 6 hours; GFR 30-50 mL/min: 50 mg every 8 hours; GFR >50 mL/min: no adjustment. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: 50 mg every 8 hours; Child-Pugh C: 25 mg every 12 hours. |
| Pediatric use | Children 1-12 years: 0.5 mg/kg intramuscularly every 6 hours; maximum 25 mg per dose. |
| Geriatric use | Elderly >65 years: initiate at 25 mg intramuscularly every 8 hours; titrate based on response and tolerability. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PHENETRON (PHENETRON).
| Breastfeeding | Phenetron is excreted in human breast milk. The M/P ratio is approximately 0.8. Caution is advised due to potential adverse effects in the infant including sedation, poor feeding, and apnea. The American Academy of Pediatrics recommends considering the risk versus benefit. |
| Teratogenic Risk | Phenetron is classified as FDA Pregnancy Category C. In the first trimester, there is a potential risk of fetal malformations based on animal studies; human data are insufficient. Second and third trimesters: Associated with increased risk of transient neonatal withdrawal syndrome, respiratory depression, and hypotonia with chronic maternal use. Avoid in late pregnancy due to risk of premature closure of the ductus arteriosus. |
■ FDA Black Box Warning
None.
| Serious Effects |
["Hypersensitivity to phenetron or any component of the formulation.","Premature neonates and newborns due to risk of severe adverse effects.","Lactation (excreted in breast milk; potential for serious adverse effects in nursing infants).","Concurrent use with monoamine oxidase inhibitors (MAOIs) as may prolong and intensify anticholinergic effects."]
| Precautions | ["May cause drowsiness; caution with driving or operating machinery.","Avoid concurrent use with CNS depressants (alcohol, sedatives).","Use with caution in patients with narrow-angle glaucoma, prostatic hypertrophy, bladder neck obstruction, or pyloroduodenal obstruction due to anticholinergic effects.","May antagonize antihypertensive drugs that act on alpha-adrenergic receptors."] |
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| Fetal Monitoring | Monitor maternal blood pressure and heart rate regularly. Assess fetal heart rate and uterine activity during labor. Consider fetal echocardiography if used in second trimester for more than 48 hours. Perform neonatal assessment for signs of withdrawal or respiratory depression after delivery. |
| Fertility Effects | Phenetron may impair female fertility by reducing ovulation rates. In males, it has been associated with decreased sperm count and motility in animal studies; human data limited. Reversible upon discontinuation. |