MALATHION
Clinical safety rating: caution
Comprehensive clinical and safety monograph for MALATHION (MALATHION).
Malathion is an organophosphate that irreversibly inhibits acetylcholinesterase, leading to accumulation of acetylcholine at cholinergic synapses, causing overstimulation of muscarinic and nicotinic receptors.
| Metabolism | Malathion is primarily metabolized by carboxylesterases and cytochrome P450 (CYP) enzymes, including CYP1A2 and CYP3A4, to malaoxon (active) and other metabolites. |
| Excretion | Primarily renal excretion of water-soluble metabolites (e.g., malathion monocarboxylic acid, dicarboxylic acid). Less than 1% excreted unchanged in urine. Biliary/fecal excretion accounts for minor elimination of metabolites. |
| Half-life | Approximately 2-3 hours for malathion (parent compound). Metabolites (e.g., malaoxon) may persist longer. Clinically, elimination is rapid, limiting systemic toxicity with acute dermal exposure. |
| Protein binding | Malathion is approximately 90-95% bound to plasma proteins (primarily albumin and alpha-1-acid glycoprotein). |
| Volume of Distribution | Approximately 3.7 L/kg (range 2-5 L/kg), indicating extensive tissue distribution. Clinical significance: High Vd suggests rapid uptake into tissues, including liver (site of activation) and adipose tissue (minor depot). |
| Bioavailability | Topical: Minimal systemic absorption (<5% through intact skin). Oral: Bioavailability unknown due to rapid presystemic metabolism (first-pass effect). Dermal: low bioavailability due to limited absorption; however, application to damaged skin increases absorption. |
| Onset of Action | Topical (scabies/pediculosis): Onset of pediculicidal/scabicidal effect within minutes (kills lice/mites). Systemic effects (if absorbed) may occur within 30 minutes to hours, but clinical onset for poisoning is variable (typically 1-4 hours). |
| Duration of Action | Topical: Single application typically eradicates lice/scabies; ovicidal activity persists for 7-10 days. Systemic effects (if absorbed) may last 12-24 hours due to acetylcholinesterase inhibition. Clinical note: Topical use rarely causes systemic toxicity unless ingested or applied to broken skin. |
0.5% lotion applied topically to entire body and left for 8-12 hours; repeat in 7-9 days if necessary.
| Dosage form | LOTION |
| Renal impairment | No dose adjustment required for GFR >30 mL/min; use with caution in GFR <30 mL/min due to potential for increased systemic absorption. |
| Liver impairment | No specific guidelines; use with caution in severe hepatic impairment due to reduced metabolism. |
| Pediatric use | Children ≥2 months: 0.5% lotion applied as a single thin layer for 8-12 hours; repeat in 7 days if live lice are present. |
| Geriatric use | No specific dose adjustment; use same as adults with caution due to age-related skin changes. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for MALATHION (MALATHION).
| Breastfeeding | Malathion is excreted into breast milk in small amounts (M/P ratio unknown). Due to low systemic absorption after topical use, risk to infant is likely low. However, caution is advised; avoid application to breast area. Use only if clearly needed. |
| Teratogenic Risk | Malathion is classified as Pregnancy Category B. Animal reproduction studies have not shown fetal risk, but no adequate human studies exist. In first trimester, theoretical risk of organophosphate exposure; however, malathion has low systemic absorption. In second and third trimesters, no specific malformations reported; avoid prolonged or repeated use. There is no evidence of teratogenicity from topical use. |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to malathion or any component of the formulation","Do not use on open wounds or inflamed skin","Avoid use in infants less than 2 months of age (due to immature skin barrier and potential systemic toxicity)"]
| Precautions | ["Flammable: avoid exposure to open flame or heat sources until hair is dry","Repeated use may increase absorption and toxicity","Avoid contact with eyes and mucous membranes","Do not use in neonates or premature infants due to increased skin permeability","Monitor for signs of cholinergic toxicity if used in large amounts or on compromised skin"] |
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| Fetal Monitoring | Monitor for maternal signs of organophosphate toxicity (e.g., headache, dizziness, nausea, blurred vision, salivation, muscle weakness). No specific fetal monitoring required unless maternal toxicity occurs. In cases of accidental ingestion or excessive use, monitor maternal plasma and RBC cholinesterase levels. |
| Fertility Effects | Malathion has not been shown to impair fertility in animal studies. No human data on fertility effects. Potential for reversible cholinesterase inhibition does not appear to affect reproductive function at clinically relevant doses. |