Malathion Side Effects: What to Expect and When to Call Your Doctor

Malathion (brand name Ovide) is an effective treatment for head lice, but like all medications, it comes with potential side effects. Most are mild and localized to the scalp — but a few important safety issues deserve your attention before your first application. This guide covers everything you need to know about malathion side effects, from what's normal to what requires a call to your doctor.

The Most Important Safety Warning: Malathion Is Flammable

Before we discuss side effects, there is one critical safety issue that isn't a side effect in the traditional sense — but is responsible for some of the most serious adverse events reported with malathion:

Malathion lotion is highly flammable. The formulation contains 78% isopropyl alcohol. While the lotion is in your hair (and while your hair is still wet), you must avoid:

Hair dryers, curling irons, flat irons, or any electric heat appliance

Open flames (candles, gas stoves, fireplaces)

Cigarettes or smoking of any kind

Always allow your hair to air-dry naturally after application and keep it uncovered. This is why many people apply malathion at bedtime — it can dry naturally overnight with minimal risk of heat exposure.

Common Side Effects of Malathion

The following side effects are most commonly reported with malathion use. They are generally mild and limited to the scalp and hair:

Scalp irritation and redness: The most commonly reported side effect. The isopropyl alcohol in the vehicle can irritate sensitive skin. If you have open sores on your scalp from scratching, irritation may be more pronounced.

Stinging sensation: A burning or stinging feeling on the scalp during application is common, especially if the skin is broken or irritated from prior scratching.

Dry hair: The alcohol base can temporarily dry out the hair shaft. This typically resolves after washing.

Strong odor: Malathion has a distinctive, somewhat unpleasant odor due to the terpineol, dipentene, and pine needle oil in the vehicle. This is a known cosmetic drawback.

Eye irritation: Accidental contact with the eyes can cause mild conjunctivitis (eye irritation). Keep eyes tightly closed during application; if contact occurs, flush immediately with water.

Serious Side Effects: Chemical Burns

The most serious adverse effect reported with malathion is chemical burns, including second-degree burns. This is uncommon but has occurred. Risk factors include:

Using more product than directed

Allowing the product to contact non-scalp skin (face, neck, ears)

Sensitive skin or broken skin from prior scratching

If skin irritation occurs during use, follow the FDA-approved guidance: discontinue use immediately, wash off the product, and wait for the irritation to clear. You may attempt re-application once the skin has healed — but if irritation recurs, stop use entirely and contact your prescriber.

What About Systemic (Whole-Body) Side Effects?

Because malathion is an organophosphate, there are theoretical concerns about systemic absorption leading to cholinergic symptoms — increased sweating, salivation, gastrointestinal symptoms, and decreased heart rate. These effects have been observed with agricultural malathion exposure, where much larger amounts are absorbed.

When used as directed — one application to the scalp — the amount absorbed through skin is minimal. Systemic toxicity from properly-used therapeutic malathion is not a clinically recognized concern. The risk increases if the product is accidentally ingested; if this occurs, call Poison Control immediately at 1-800-222-1222.

When to Call Your Doctor

Contact your prescriber if:

You develop significant skin irritation, burning, or visible skin damage after application

Eye irritation persists after flushing with water

Live lice are still present 7-9 days after your second application

You experience any signs of allergic reaction: hives, difficulty breathing, facial swelling

For information about potential drug interactions with malathion, see our guide: Malathion Drug Interactions: What to Avoid and What to Tell Your Doctor.