Permethrin lasts anywhere from a few hours on your skin to several months on indoor surfaces, depending entirely on how and where it’s being used. On treated clothing, it can survive up to 70 washes. On your body after a scabies or lice treatment, it stays active for about 10 days before your body fully eliminates it within a week. Here’s a breakdown for each common use.
On Skin for Scabies Treatment
Permethrin 5% cream is designed to be left on the skin for 8 to 14 hours, then washed off in the shower. During that window, the active ingredient penetrates the outer layer of skin enough to kill scabies mites on contact. Very little actually enters your bloodstream: roughly 0.5% of the applied dose gets absorbed, based on studies measuring metabolites in urine.
A single application is often enough, but because mite eggs can survive the first treatment and hatch days later, the CDC recommends a second application about one week after the first if needed. That one-week spacing is timed to catch newly hatched mites before they can mature and reproduce.
On Hair for Lice Treatment
Permethrin 1% (the over-the-counter lice rinse) stays detectable on hair for at least 10 days after a single application. This residual activity is one of its advantages: it continues killing newly hatched lice (nymphs) that emerge from eggs the initial treatment didn’t destroy. That’s why a second treatment is typically recommended 7 to 10 days later, catching any remaining nymphs during the gap between hatching and the point where they could lay new eggs.
On Treated Clothing and Gear
This is where permethrin’s longevity varies the most, and the difference comes down to how the treatment was applied.
Factory-treated clothing, where permethrin is bonded to the fabric during manufacturing, can remain effective for up to 70 wash cycles. That translates to roughly a year or more of regular use for most people. Brands like Insect Shield and similar lines use this bonding process.
DIY spray treatments, where you apply permethrin from an aerosol or pump bottle and let it dry on your clothes, last about six washes. That’s enough for a camping trip or a few weeks of outdoor work, but you’ll need to reapply regularly if you’re relying on it throughout a full tick or mosquito season.
On Indoor Surfaces
Permethrin is remarkably persistent indoors. In a study that tracked permethrin applied to surfaces and kept in a test house, the insecticide maintained up to 89.6% of its original concentration after 112 days (about four months). Without direct sunlight or heavy rain to break it down, indoor permethrin degrades slowly. This is why it’s a common choice for baseboards, window frames, and other areas where crawling insects travel. It’s also why residues can linger in homes long after a professional pest treatment.
How Sunlight and Washing Affect Longevity
Outdoors, the two biggest factors that shorten permethrin’s life are washing and sun exposure, in that order. Research on permethrin-treated bed nets found that washing caused far more loss of effectiveness than sunlight did. Nets exposed to continuous sunlight for 15 to 45 days between washes showed significantly reduced activity, but short sun exposure of a few hours (like drying laundry on a line) caused no measurable harm.
Natural pyrethrins, the plant-based compounds permethrin was modeled after, break down rapidly in UV light. Synthetic permethrin was engineered to resist this, but prolonged direct sunlight still degrades it over time. The practical takeaway: if you’re treating outdoor gear, store it out of direct sun when not in use, and expect to reapply more often than you would for indoor items.
How Long It Stays in Your Body
After a skin application, your body fully eliminates permethrin metabolites within about one week. The process is straightforward: the small amount that gets absorbed is broken down by your liver into metabolites that are filtered out through urine. In studies of oral exposure (which results in much higher absorption than skin contact), those metabolites peaked in the blood around 7 hours and were eliminated with a half-life of roughly 5 to 7 hours.
The low absorption rate from skin, around 0.5% to 2% of the applied dose, is a key reason permethrin is considered safe for topical use in adults, children as young as two months old, and even during pregnancy in many guidelines. Your body processes and clears the tiny absorbed amount efficiently.
Quick Reference by Use
- Scabies cream (5%): Left on skin 8 to 14 hours, second dose one week later if needed
- Lice rinse (1%): Active on hair for at least 10 days
- Factory-treated clothing: Up to 70 washes
- DIY spray-treated clothing: About 6 washes
- Indoor surfaces: Up to 4 months or longer at near-full strength
- In your body after skin application: Fully cleared within 1 week