Malathion
Scabicides and pediculicides
Scabicides and pediculicides
An organophosphate lotion that kills head lice and their eggs by blocking their acetylcholinesterase. It is flammable: no hair dryers or flames.
Quick facts
- Group
- Scabicides and pediculicides
- Target
- Acetylcholinesterase of the louse
- Used for
- Head lice and their eggs
- Key effects
- Scalp and skin irritation
- Monitor
- Live lice one week · Scalp irritation or burns
- ATC
- P03AX03
No doses: the clinician sets them by the current guideline.
Mechanism of action
Malathion is an organophosphate cholinesterase inhibitor. In the louse it blocks acetylcholinesterase, acetylcholine builds up at nerve synapses, and the louse is paralysed and dies; it also kills eggs (ovicidal). People are protected because plasma carboxylesterases hydrolyse and inactivate malathion much faster in humans than in insects. If swallowed or heavily absorbed it can still cause cholinergic poisoning.
- Malathion inhibits the louse's acetylcholinesterase
- Acetylcholine accumulates at its synapses
- Lice and their eggs are paralysed and die
- Human carboxylesterases inactivate it far faster than insects
Indications
- Head lice and their eggs in children six years and older and adults Pediculosis (head, body and pubic lice) →
- Alternative for scabies where permethrin fails (aqueous lotion) Scabies →
Adverse effects
Common
- Scalp and skin irritation, stinging
- Mild conjunctivitis if it reaches the eyes
Contraindications
- Neonates and infants (more permeable scalp)
- Hypersensitivity to malathion or the vehicle
Key interactions
- Open flames, hair dryers, curling irons, cigarettes
- The alcohol-based lotion and wet hair can catch fire.
What is monitored
- Live lice one week or more after treatment, which may need a second application
- Scalp irritation or burns
- Signs of cholinergic excess after accidental ingestion
Administration and safety
- Applied to dry hair and scalp only, eyes closed, behind the ears and the back of the neck included.
- Hair is left to dry naturally and uncovered; no hair dryers, flames or smoking until it is washed out.
- Shampooed out over a sink after the set time, then combed with a nit comb.
- Helpers wash their hands after application and rinsing.
Pregnancy and breastfeeding
Animal studies showed no teratogenicity; human data are limited, so it is used in pregnancy only after medical advice.
Nursing care
- Check the child's age before use.
- Teach the fire risk clearly to families.
- Teach to flush the eyes with water if splashed and to seek care if irritation persists.
- Keep the bottle away from children: swallowing it can cause organophosphate poisoning.
Pharmacokinetics
- Absorption
- Some is absorbed through the skin; infants' scalps are more permeable.
- Metabolism
- Rapid hydrolysis by plasma carboxylesterases in humans.
Test yourself
Think of the answer, then tap the question to see it.
How does malathion kill lice, and why are humans relatively spared?
It inhibits the louse's acetylcholinesterase; human plasma carboxylesterases inactivate it much faster than insects can.
What is the main practical safety warning with malathion lotion?
It is flammable: no flames, hair dryers, curling irons or smoking while the hair is wet.
What antidote is used if malathion is swallowed and causes cholinergic poisoning?
Atropine, with supportive care for respiratory distress.
Connections
← Back to the class: Scabicides and pediculicides
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Malathion Topical: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - MALATHION lotion (DailyMed (U.S. National Library of Medicine), 2026); Scabies: Diagnosis and Treatment with Images (DermNet, 2026).
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Representative model, for educational purposes only.