Topical organophosphate antiparasitic Pregnancy: US label §Pregnancy (Pregnancy Category B), verbatim: 'There was no evidence of teratogenicity in studies in rats and rabbits at doses up to 900 mg/kg/day and 100 mg/kg/day malathion, respectively. A study in rats failed to show any gross fetal abnormalities attributable to feeding malathion up to 2,500 ppm (~ 200 mg/kg/day) in the diet during a three - generation evaluation period. These doses were approximately 40 to 180 times higher than the dose anticipated in a 60 kg adult (based on body surface area and assuming 100% bioavailability). Because animal reproduction studies are not always predictive of human responses, this drug should be used (or handled) during pregnancy only if clearly needed.' This summary may be incomplete — check the full SPC.

Malathion

Brand names: Derbac-M, Quellada M

Malathion is a topical organophosphate insecticide used for head lice and for scabies as an alternative to permethrin.

Adult dose

Dose: Head lice — apply the lotion to DRY hair in an amount just sufficient to thoroughly wet the hair and scalp, paying particular attention to the back of the head and neck; allow the hair to dry naturally and remain uncovered; after 8 to 12 hours shampoo the hair, rinse, and use a fine-toothed (nit) comb to remove dead lice and eggs.

Route: Topical — lotion to dry hair and scalp. Wash hands after applying to the scalp. Allow hair to dry naturally: 'use no electric heat source, and allow hair to remain uncovered.'

Frequency: Single application. 'If lice are still present after 7 - 9 days, repeat with a second application of OVIDE Lotion. Further treatment is generally not necessary.'

Max: No maximum quantity is stated in the fetched label — the dose is qualitative ('an amount just sufficient to thoroughly wet the hair and scalp'). The only limit expressed is on repeats: a second application at 7-9 days if lice persist, after which 'Further treatment is generally not necessary.' That is a general statement, not an explicit ceiling, so it is not published as one.

SCOPE: this covers HEAD LICE only, which is this page's primary indication (its clinical pearls lead with head lice and describe scabies as the second-line use 'if permethrin unsuitable'). The fetched bundle contains NO scabies dose at all. || Verbatim §DOSAGE AND ADMINISTRATION: 'Apply OVIDE Lotion on DRY hair in amount just sufficient to thoroughly wet the hair and scalp. Pay particular attention to the back of the head and neck while applying OVIDE Lotion. Wash hands after applying to scalp. Allow hair to dry naturally - use no electric heat source, and allow hair to remain uncovered. After 8 to 12 hours, the hair should be shampooed. Rinse and use a fine - toothed (nit) comb to remove dead lice and eggs. If lice are still present after 7 - 9 days, repeat with a second application of OVIDE Lotion. Further treatment is generally not necessary. Other family members should be evaluated by a physician to determine if infested, and if so, receive treatment.' || US LABEL, TWO CAVEATS FOR UK USE. (a) Contact time: this label says 8 to 12 hours; the figure published above is the source figure, and the UK SPC should be checked before it is used in the UK. (b) Strength and vehicle: no strengths section was fetched, so the concentration of this lotion is not stated in the bundle, and UK malathion products differ from the US one in vehicle (alcoholic versus aqueous), which affects both efficacy and the flammability/asthma warnings. || Flammability is not mentioned anywhere in the fetched sections despite 'use no electric heat source' — do not assume the bundle carries the full warning set.

Paediatric dose

Route: Topical — lotion to dry hair and scalp, applied as for adults

Frequency: As for adults — a single application, with a second application at 7-9 days only if lice are still present.

Max: No maximum is stated for children in the fetched label.

NOT weight-based — this is a topical 'quantity sufficient' application, so dosePerKg is null and no calculator figure applies. The label gives no separate paediatric dose, only restrictions. Verbatim §CONTRAINDICATIONS: 'OVIDE Lotion is contraindicated for neonates and infants because their scalps are more permeable and may have increased absorption of malathion.' Verbatim §Pediatric Use: 'The safety and effectiveness of OVIDE Lotion in children less than 6 years of age has not been established via well-controlled trials.' For children, check a children's formulary and the UK product label for the age from which the specific UK preparation is licensed.

Dose adjustments

Renal

Not stated — no renal-impairment statement appears in any fetched section of this label.

Hepatic

Not stated — no hepatic-impairment statement appears in any fetched section of this label.

Dose independently re-sourced from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Neonates and infants — verbatim: 'OVIDE Lotion is contraindicated for neonates and infants because their scalps are more permeable and may have increased absorption of malathion.'
Known sensitivity to malathion or to any ingredient of the vehicle — verbatim: 'OVIDE Lotion should also not be used on individuals known to be sensitive to malathion or any of the ingredients in the vehicle.'

Side effects

Irritation of the skin and scalp — verbatim: 'Malathion has been shown to be irritating to the skin and scalp.'
Chemical burns, including second-degree burns — verbatim: 'Other adverse reactions reported are chemical burns including second-degree burns.'
Mild conjunctivitis following accidental eye contact — verbatim: 'Accidental contact with the eyes can result in mild conjunctivitis.'
Contact allergic sensitisation — potential not established; verbatim: 'It is not known if OVIDE Lotion has the potential to cause contact allergic sensitization.'

Clinical monograph

How it works

It is acetylcholinesterase inhibitor that is toxic to lice and mites, and aqueous formulations also have ovicidal activity against louse eggs.

Prescribing in practice

Aqueous (water-based) formulations are preferred for the face, scalp, broken skin and in asthma, as alcoholic lotions can irritate and may trigger wheezing.
Apply to the affected area, leave for the recommended contact time and then wash off; a second application is usually advised to kill newly hatched lice.
For scabies the whole body is treated, and close contacts should be treated simultaneously to prevent reinfestation.

Monitoring

Check for treatment success by detection combing for head lice or symptom resolution for scabies, and assess the skin for irritation.

Counselling the patient

Follow the recommended contact time before washing the product off.
Apply a second treatment after about a week to catch newly hatched lice.
Treat all affected household contacts at the same time.

Evidence & guidelines

Malathion is an established recommended treatment for head lice and scabies, used as an alternative to permethrin where resistance or intolerance occurs.

Reference: NICE CKS; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Pathways