Malathion (Monograph)

Brand name: Ovide
Drug class: Scabicides and Pediculicides
VA class: AP300
Chemical name: [(Dimethoxyphosphinothioyl)-thio]-butanedioic acid diethyl ester
Molecular formula: C₁₀H₁₉O₆PS₂
CAS number: 121-75-5

Introduction

Pediculicide; organophosphate anticholinesterase insecticide.

Uses for Malathion

Pediculosis

Topical treatment of pediculosis capitis (head lice infestation) in adults and children ≥6 years of age.

Permethrin 1% generally is considered the treatment of choice; malathion 0.5% is recommended when permethrin resistance is suspected.

Base selection of a pediculicide on efficacy (including both pediculicidal and ovicidal activity), safety, cost, availability, ease of application, age of patient, presence of other scalp infections, patient preference, severity of the infestation, potential for transmission, number of recurrences, and the pattern of resistance in the geographic region.

Has been recommended as an alternative for treatment of pediculosis pubis^{† [off-label]} (pubic lice infestation). Although CDC states malathion 0.5% may be used for treatment of pediculosis pubis resistant to other pediculicides, the malathion preparation commercially available in the US is formulated in a vehicle containing isopropyl alcohol 78% and should not be used for treatment of pediculosis pubis since the vehicle may cause burning and irritation of excoriated skin and genitalia.

Topical treatment of pediculosis corporis^{† [off-label]} (body lice infestation). In some cases, body louse infestations may be treated by improved hygiene and by decontaminating clothes and bedding by washing at temperatures that kill lice. If the infestation is severe, a pediculicide also should be used (e.g., topical permethrin, topical pyrethrins with piperonyl butoxide, topical malathion, oral ivermectin).

Scabies

Has been used for topical treatment of scabies^{† [off-label]} (mite infestation), usually as an aqueous lotion. However, the malathion preparation commercially available in the US is formulated in a vehicle containing isopropyl alcohol 78% and should not be used for treatment of scabies since the vehicle may cause burning and irritation of excoriated skin and genitalia.

Malathion Dosage and Administration

General

Measures to Avoid Reinfestation and Transmission

• To avoid reinfestation or transmission of pediculosis, most experts recommend that clothing and bed linen that may have been contaminated by the infested individual during the 2 days prior to treatment should be decontaminated (machine-washed in hot water and dried in a hot dryer, dry-cleaned, or treated with an appropriate pesticide) or removed from body contact for ≥72 hours.

• Items that cannot be laundered or dry-cleaned should be removed from contact and sealed in a plastic bag for 10–14 days.

• Combs and brushes used by the infected patient may be disinfected by soaking in hot water (temperature >54°C) for 5–10 minutes; alternatively, they can be soaked in alcohol or a pediculicide for 1 hour.

• Furniture and floors of rooms inhabited by patients infested with lice should be thoroughly vacuumed. Fumigation of living areas is not necessary and is not recommended.

• In the treatment of pediculosis capitis (head lice infestation), a fine-toothed comb often is recommended to remove any remaining nits (eggs) or nit shells. Some clinicians do not consider nit removal necessary since only live lice can be transmitted, but recommend it for aesthetic reasons and to decrease diagnostic confusion and unnecessary retreatment. Others strongly recommend removal of nits (especially those within 1 cm of the scalp) since no pediculicide is 100% ovicidal and potentially viable nits may remain on the hair after pediculicide treatment. Although many schools will not allow children with nits to attend, AAP and other experts consider these no-nit policies excessive.

Administration

Topical Administration

Apply topically to scalp hair as a 0.5% lotion.

For external use only. Do not apply topically to skin, eyes, or pubic area and do not administer orally.

Avoid contact with the eyes. Eyes should be closed tightly and covered with a soft towel or washcloth while the lotion is applied to or washed off of scalp hair. If contact with the eyes occurs, immediately flush with water. If eye irritation persists or if visual changes occur, consult a clinician.

Apply to dry hair; after application, leave hair uncovered and allow to dry naturally.

Malathion 0.5% lotion is flammable. Do not expose the lotion or hair wetted with the lotion to an open flame, lighted cigarette, or electric heat source (e.g., hair dryer, electric curlers). Individuals applying the lotion or patients whose hair is wet with the lotion should refrain from smoking.

Individuals applying malathion 0.5% lotion should wash their hands thoroughly after application is completed.

Dosage

Pediatric Patients

Pediculosis

Pediculosis Capitis (Head Lice Infestation)

Topical

Children ≥6 years of age: Apply to dry scalp hair in an amount sufficient to thoroughly wet the hair and scalp (including areas on the back of the head and nape of the neck). After 8–12 hours, wash hair (including areas on the back of the head and neck) with shampoo and rinse with water. Use a fine-tooth (nit) comb to remove dead lice and nits.

One treatment usually is successful; treatment may be repeated after 7–10 days if live lice still are present. Additional treatments generally are unnecessary.

Adults

Pediculosis

Pediculosis Capitis (Head Lice Infestation)

Topical

Apply to dry scalp hair in an amount sufficient to thoroughly wet the hair and scalp (including areas on the back of the head and nape of the neck). After 8–12 hours, wash hair (including areas on the back of the head and neck) with shampoo and rinse with water. Use a fine-tooth (nit) comb to remove dead lice and nits.

One treatment usually is successful; treatment may be repeated after 7–10 days if live lice still are present. Additional treatments generally are unnecessary.

Cautions for Malathion

Contraindications

History of hypersensitivity to malathion or any ingredient in the formulation.

Infants and neonates. (See Pediatric Use under Cautions.)

Warnings/Precautions

Warnings

Flammability

Commercially available 0.5% lotion is flammable.

Do not expose malathion 0.5% lotion or hair wet with the lotion to an open flame, lighted cigarette, or electric heat source (e.g., hair dryers, electric curlers). After application, leave the hair uncovered and allow to dry naturally.

Individuals applying the lotion and those with hair wet with the lotion should refrain from smoking.

Sensitivity Reactions

Dermatologic and Sensitivity Reactions

Potential for malathion 0.5% lotion to cause contact allergic sensitization unknown. Dermatitis of the scalp has been reported with topical malathion 5% lotion (10 times the usually recommended dosage). Contact dermatitis has been reported in individuals exposed to agricultural formulations of malathion.

May be irritating to the skin and scalp; slight stinging sensation may occur after application of malathion 0.5% lotion. If skin irritation occurs, immediately remove the lotion by washing scalp and hair.

After irritation clears, lotion may be reapplied. If irritation recurs, consult a clinician.

General Precautions

Administration Precautions

Avoid contact with the eyes; mild conjunctivitis may occur. Do not use for treatment of pediculosis of the eyelashes.

Eyes should be closed tightly and covered with a soft towel or washcloth while the lotion is applied to or washed off of scalp hair.

If accidental contact with the eyes occurs, the affected eye(s) should be flushed thoroughly with water.

Specific Populations

Pregnancy

Category B.

Lactation

Not known whether malathion is distributed into human milk. Malathion was not detected in milk samples (minimum limits of detection <5 mcg/L) from nursing women who resided in geographic areas that received extensive aerial spraying with an agricultural formulation of malathion for insect control.

Use caution when malathion 0.5% lotion is administered to or handled by a nursing woman.

Pediatric Use

Safety and efficacy not established in children <6 years of age. Contraindicated in infants and neonates because their scalps are more permeable and increased absorption of malathion may occur.

Keep out of reach of children; use on children only under the direct supervision of an adult.

Children should be warned to stay away from lighted cigarettes, open flames, and electric heat sources while their hair is wet with the lotion. (See Flammability under Cautions.)

Some clinicians recommend that the malathion preparation commercially available in the US not be used on small children or in children with asthma to prevent exposure to fumes from the isopropyl alcohol vehicle.

Common Adverse Effects

Irritation of skin and scalp, stinging or burning.

Malathion Pharmacokinetics

Absorption

Bioavailability

Systemic absorption of malathion following topical application of the 0.5% lotion commercially available in the US has not been specifically studied to date.

Results of a study in healthy adults using several different aqueous- and alcohol-based formulations of topical malathion 0.5% (not the preparation commercially available in the US) indicate small amounts of the drug are absorbed following application to the scalp and the extent of absorption is similar between aqueous and alcoholic formulations.

Malathion is absorbed systemically following ingestion, topical application to skin and mucous membranes, or inhalation of dusts or aerosols.

Distribution

Extent

Not known whether malathion crosses the placenta following topical application.

Not known whether malathion is distributed into milk following topical application. Not detected in milk samples (minimum limits of detection <5 mcg/L) from nursing women who resided in geographic areas that received extensive aerial spraying with an agricultural formulation of malathion for insect control.

Elimination

Metabolism

Rapidly metabolized in vivo, principally by hydrolysis of the carboxyl ester linkage to inactive metabolites by carboxylesterases.

Elimination Route

Malathion and its metabolites are excreted in urine; the monocarboxylic and dicarboxylic acid metabolites of malathion also are excreted in bile.

In one study following topical application to the scalp of aqueous- or alcohol-based preparations of malathion (not the preparation commercially available in the US), approximately 0.2–3.2% of the applied malathion dose was eliminated in the urine as metabolites within 96 hours.

Stability

Storage

Topical

Lotion

20–25°C.

Do not expose to an open flame, lighted cigarette, or heat source (e.g., hair dryer, electric curlers).

Actions and Spectrum

• An organophosphate anticholinesterase insecticide used as a pediculicide.

• Active in vitro and in vivo against Pediculus humanus var. capitis (head louse) and its nits (eggs) and Phthirus pubis (pubic or crab louse). Also active against Sarcoptes scabiei, the causative agent of scabies.

• Therapeutic failure and resistance to malathion has been reported in P. humanus var. capitis and some strains of P. humanus var. corporis (body louse) in some areas of the world (e.g., United Kingdom, France, Israel). Clinical importance of these reports as it relates to treatment of lice infections in the US is unclear; prevalence of malathion resistance in the US has not been determined.

• May be active against some P. capitis resistant to permethrin and pyrethrins.

Advice to Patients

• Importance of using only as directed.

• Advise patients of personal protective measures to avoid reinfestation or transmission of lice.

• Advise patients that malathion 0.5% lotion is flammable. Importance of staying away from open flames, lighted cigarettes, and electric heat sources (e.g., hair dryers, electric curlers) while the hair is wet with the lotion. Importance of not smoking while applying the lotion or while hair is wet with the lotion.

• Importance of thoroughly washing hands after applying malathion 0.5% lotion.

• Advise patients to avoid contact of malathion 0.5% lotion with the eyes. If accidental contact with the eyes occurs, immediately flush the affected eye(s) with water. If eye irritation persists or if visual changes occur, consult a clinician.

• Importance of immediately consulting clinician and/or poison control center if malathion 0.5% lotion is accidentally ingested.

• Advise patients that a slight stinging sensation may occur when malathion 0.5% lotion is applied. If skin irritation occurs, immediately wash lotion from scalp and hair. After irritation clears, the lotion can be reapplied. If irritation reoccurs, consult a clinician.

• Importance of informing clinician of existing or contemplated concomitant therapy, including prescription and OTC drugs.

• Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.

• Importance of informing patients of other important precautionary information. (See Cautions.)

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

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