BMA Concise Guide to Medicine & Drugs


Brand names Derbac-M, Prioderm, Quellada M

Used in the following combined preparations None


Drug group Drug to treat skin parasites

Overdose danger rating Low

Dependence rating Low

Prescription needed No

Available as generic No


Malathion is an organophosphate insecticide used in the treatment of lice and mite infestations. The drug kills parasites by interfering with their nervous system function, causing paralysis and death.

Malathion is applied topically, either as a shampoo or a lotion. Lotion is more convenient to use than shampoo, requiring only a single application. It is also more effective because shampoo is diluted in use. Lotions with a high alcohol content are unsuitable for small children or asthmatics, who may be affected by the solvent, or for treating crab lice in the genital area. However, the water-based liquid is suitable. Care should be taken to avoid contact of the drug with the eyes or broken skin.

If resistance occurs during a course of treatment, your pharmacist will recommend an alternative insecticide such as permethrin. Treatment with malathion will not prevent infestation.


Follow instructions on the label. Call your doctor if symptoms worsen.

How taken/used Topical liquid, lotion, shampoo.

Frequency and timing of doses Scabies Once only (lotion or topical liquid). Lice 3 applications 3 days apart (shampoo); 2 doses, 7 days apart (lotion or topical liquid).

Adult dosage range As directed. Family members and close contacts should also be treated.

Onset of effect Lotion or topical liquid leave on for 12 hours (lice), or 24 hours (scabies), before washing off. Shampoo leave on for 5 minutes, rinse off, repeat, then use fine-toothed comb.

Duration of action Until washed off.

Diet advice None.

Storage Keep in original container at room temperature out of the reach of children. Protect from light.

Missed dose When a repeat application of the shampoo has been missed, it should be carried out as soon as is practicable.

Stopping the drug Malathion should be applied as a single application or as a short course of treatment.

Exceeding the dose An extra application is unlikely to cause problems. Take emergency action if the insecticide has been swallowed.


Used correctly, malathion preparations are unlikely to produce adverse effects, although the alcoholic fumes given off by some lotions may cause wheezing in people with asthma. Rarely, malathion may cause skin irritation; consult your doctor if this is severe.




Be sure to consult your doctor or pharmacist before taking this drug if:

· You have asthma.

Pregnancy No evidence of risk. It is unlikely that enough malathion would be absorbed after occasional application to affect the developing fetus.

Breast-feeding No evidence of risk. It is unlikely that enough malathion would be absorbed after occasional application to affect the baby.

Infants and children No special problems, but seek medical advice for infants under 6 months.

Over 60 No special problems.

Driving and hazardous work No special problems.

Alcohol No special problems.


Malathion is intended for intermittent use only. The lotions should not be used more than once a week for three weeks at a time. If there is a need to use malathion more frequently, it is possible that resistance has built up; seek your doctor’s advice.