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 |
| Molecular Weight | 347.45 |
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 | Avoid during first trimester due to teratogenic effects observed in animal studies; may cause fetal malformations. |
| 2nd trimester | Use only if clearly needed; risk of fetal toxicity and growth restriction. |
| 3rd trimester | Contraindicated in third trimester due to risk of premature closure of ductus arteriosus and neonatal complications. |
Clinical note
Comprehensive clinical and safety monograph for PHENETRON (PHENETRON).
| Placental transfer | Placental transfer occurs with measurable fetal concentrations; crosses placenta readily with fetal/maternal ratio ~0.8. |
| Breastfeeding | Phenetron is excreted into breast milk; may cause adverse effects in nursing infants including drowsiness, irritability, and feeding difficulties. Decide if to discontinue nursing or drug based on importance to mother. |
■ FDA Black Box Warning
None.
| Serious Effects |
History of hypersensitivity to phenetron or any componentSevere hepatic impairmentThird trimester pregnancyConcurrent use with MAO inhibitors
| 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. |
| Food/Dietary | Avoid grapefruit and grapefruit juice as they may increase phenylephrine levels. Avoid alcohol as it enhances sedation and risk of adverse effects. No significant interactions with other foods. |
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| Lactation Rating |
| L4 (Possibly Hazardous) |
| 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. |
| 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. |
| Clinical Pearls | PHENETRON is a combination of phenylephrine and chlorpheniramine. Monitor blood pressure in hypertensive patients due to phenylephrine. Avoid in patients with narrow-angle glaucoma or urinary retention. Onset of action is 15-30 minutes; duration 4-6 hours. |
| Patient Advice | Take this medication exactly as directed; do not exceed recommended dose. · Avoid driving or operating machinery until you know how this medication affects you, as drowsiness may occur. · Do not take with other medications containing phenylephrine or antihistamines. · Notify your doctor if you have high blood pressure, heart disease, diabetes, thyroid problems, or prostate issues. · Limit alcohol intake as it may increase drowsiness and other side effects. |