Stop Lice Maximum Strength Side Effects: What to Expect and When to Call Your Doctor

Stop Lice Maximum Strength — which contains pyrethrin 0.33% and piperonyl butoxide 4% — is an FDA-approved OTC treatment with a well-established safety profile. Most people tolerate it without significant problems. However, knowing what side effects to expect — and which ones require medical attention — helps you use this product safely.

Common Side Effects: What's Normal

The following side effects are common and typically resolve on their own. They do not indicate that the medication is unsafe or that treatment has failed:

• Scalp itching: Itching is already a symptom of lice infestation and may temporarily worsen immediately after treatment. This is a normal response to the dying lice and the product itself. Itching can persist for up to 7–10 days after successful treatment and does not mean the lice are still alive.
• Scalp redness: Mild redness at the application site is common. This typically resolves within hours to a day.
• Mild burning or stinging: A mild burning or stinging sensation during or briefly after application is not unusual. Remove the product from your eyes immediately if contact occurs.
• Tingling or numbness: A temporary tingling sensation on the scalp is sometimes reported. This is generally mild and short-lived.

Serious Side Effects: When to Seek Help Immediately

Serious reactions to Stop Lice Maximum Strength are rare but can occur. Seek emergency medical care immediately if you experience:

• Signs of a severe allergic reaction (anaphylaxis): Hives, widespread rash, swelling of the face, lips, tongue, or throat, difficulty breathing, dizziness, or chest tightness. Call 911 immediately.
• Eye irritation that doesn't resolve: If the product gets in the eyes and flushing with water doesn't relieve irritation within a few minutes, seek medical attention. Call Poison Control at 1-800-222-1222.
• Swallowing the product: If swallowed, call Poison Control at 1-800-222-1222 immediately. Symptoms of ingestion may include nausea, vomiting, diarrhea, or unusual drowsiness.
• Signs of skin infection: Persistent scratching can cause skin breaks that become infected. If you notice increasing redness, warmth, pus, or worsening pain at scratch sites, contact your doctor.

Who Is Most at Risk for Side Effects?

Certain individuals are at higher risk of reactions or should consult a doctor before using Stop Lice Maximum Strength:

• People allergic to ragweed or chrysanthemums: Pyrethrins are derived from chrysanthemum flowers. Cross-reactivity can occur in people with ragweed or chrysanthemum allergies, potentially causing breathing difficulty or asthmatic episodes. Do not use this product if you have these allergies.
• Children under 2 years: Do not use without medical advice. The CDC recommends only manual nit and lice removal for children under 2.
• Pregnant or breastfeeding individuals: Consult a healthcare provider before using. Systemic absorption from topical application is minimal, but a healthcare provider should confirm this is appropriate for your specific situation.
• Irritated or broken skin on the scalp: Do not use if the scalp is already significantly irritated, injured, or infected — this can increase absorption and worsen irritation.

Safety Tips to Minimize Side Effects

Following these precautions reduces the risk of irritation or adverse reactions:

• Apply in a well-ventilated room to reduce inhalation exposure
• Protect eyes with a washcloth or towel during application
• Do not leave the product on longer than 10 minutes — follow label directions exactly
• Do not use near open flames, candles, or heat sources — the product contains flammable ingredients
• Keep out of reach of children after use — store securely
• Do not apply to eyebrows or eyelashes — consult a doctor if lice are found in those areas

When Should I Call My Doctor After Using Stop Lice Maximum Strength?

Call your doctor if: side effects persist or worsen after 24–48 hours; you see live lice still moving 24 hours after treatment (possible resistance — see our guide on drug interactions and what to tell your doctor); or any sign of skin infection develops. Call 911 or go to the emergency room immediately for signs of anaphylaxis.