PYRILAMINE MALEATE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PYRILAMINE MALEATE (PYRILAMINE MALEATE).
Pyrilamine is a first-generation antihistamine that competitively antagonizes histamine at H1 receptors, thereby preventing histamine-mediated effects such as increased vascular permeability, vasodilation, and bronchoconstriction.
| Metabolism | Hepatic metabolism primarily via CYP450 enzymes, including CYP2D6 and CYP3A4. |
| Excretion | Primarily renal as metabolites; about 80-90% excreted in urine within 24 hours, with less than 5% unchanged; minor biliary/fecal elimination. |
| Half-life | Approximately 16-23 hours in healthy adults; may be prolonged in elderly or hepatic impairment. |
| Protein binding | Approximately 98-99% bound to serum proteins, primarily albumin. |
| Volume of Distribution | Approximately 5-10 L/kg; indicates extensive tissue distribution. |
| Bioavailability | Oral: approximately 70-90% due to first-pass metabolism; parenteral: 100%. |
| Onset of Action | Oral: 30-60 minutes; parenteral: 10-20 minutes. |
| Duration of Action | Oral: 4-6 hours; parenteral: 3-4 hours; clinical effects may last longer due to active metabolites. |
| Molecular Weight | 401.5 |
| Action Class | First-generation antihistamine (H1-receptor antagonist) |
25-50 mg orally every 6-8 hours as needed, not to exceed 200 mg per day.
| Dosage form | TABLET |
| Renal impairment | eGFR 30-50 mL/min: administer every 12 hours; eGFR 15-29 mL/min: administer every 12-18 hours; eGFR <15 mL/min: administer every 24 hours or avoid use. |
| Liver impairment | Child-Pugh Class A: no adjustment needed; Child-Pugh Class B: reduce dose by 50%; Child-Pugh Class C: avoid use. |
| Pediatric use | Children 2-6 years: 6.25 mg (1/4 of 25 mg tablet) orally every 6 hours as needed; Children 6-12 years: 12.5 mg (1/2 of 25 mg tablet) orally every 6 hours as needed; maximum 75 mg per day for ages 6-12. |
| Geriatric use | Initiate at 12.5 mg orally every 12 hours; increase slowly as tolerated due to increased risk of sedation, confusion, and anticholinergic effects. |
| 1st trimester | Avoid use due to potential teratogenic effects; limited human data, animal studies show adverse effects. |
| 2nd trimester | Use only if clearly needed; may cause uterine contractions or fetal distress. |
| 3rd trimester | Avoid near term due to risk of respiratory depression or withdrawal symptoms in neonate. |
Clinical note
Comprehensive clinical and safety monograph for PYRILAMINE MALEATE (PYRILAMINE MALEATE).
| Placental transfer | Crosses the placenta; degree not well quantified but expected due to low molecular weight and lipophilicity. |
| Breastfeeding | Excreted into breast milk in low amounts; however, due to potential for sedation or irritability in infant, use with caution, especially in neonates or preterm infants. Monitor for signs of sedation or poor feeding. |
■ FDA Black Box Warning
None.
| Common Effects | Drowsiness, Sedation, Dizziness, Dry mouth, Blurred vision, Urinary retention |
| Serious Effects | QT prolongation and torsades de pointes, Seizures, Acute dystonic reactions, Severe hypotension, Anaphylaxis |
Hypersensitivity to pyrilamine or any componentConcurrent therapy with MAOIsSevere liver diseaseNarrow-angle glaucomaUrinary retentionBreastfeeding in preterm or high-risk infants
| Precautions | CNS depressant effects: May cause drowsiness and impair mental/physical abilities; avoid concurrent use with alcohol or other CNS depressants., Anticholinergic effects: Caution in patients with asthma, increased intraocular pressure, hyperthyroidism, cardiovascular disease, or urinary retention., Use in elderly: Increased risk of dizziness, sedation, and confusion. |
| Food/Dietary |
Loading safety data…
| Lactation Rating |
| L3 |
| Teratogenic Risk | First trimester: No adequate human studies; animal studies not reported. Potential anticholinergic effects. Second and third trimesters: Limited data; theoretical risk of neonatal anticholinergic syndrome if used near term. Avoid use in pregnancy unless benefit outweighs risk. |
| Fetal Monitoring | Monitor maternal heart rate, blood pressure, and urinary retention; fetal heart rate monitoring if used near term. |
| Fertility Effects | No specific studies on human fertility; anticholinergic effects may theoretically affect cervical mucus or sperm transport. |
| No significant food interactions; however, alcohol and grapefruit juice may enhance sedative effects and should be avoided. |
| Clinical Pearls | Pyrilamine maleate is a first-generation antihistamine with strong anticholinergic effects; use cautiously in elderly due to risk of confusion, urinary retention, and falls. Onset is rapid (15-30 min) but duration short (4-6 h), making it useful for acute allergic reactions but not for chronic therapy. Sedation is prominent; avoid concurrent CNS depressants. Contraindicated in narrow-angle glaucoma, prostatic hypertrophy, and breastfeeding. |
| Patient Advice | This medication may cause drowsiness; do not drive or operate heavy machinery until you know how it affects you. · Avoid alcohol and other sedatives while taking this drug. · Take exactly as prescribed; do not exceed recommended dose. · If you have glaucoma, difficulty urinating, or an enlarged prostate, consult your doctor before use. · Do not use in children under 6 years unless directed by a physician. · Stop use and seek medical help if you experience rapid heartbeat, hallucinations, or severe dizziness. |