Ivermectin's plasma half-life in humans is approximately 18 hours, though published estimates range from about 12 to 36 hours depending on the study population and assay. Applying the standard pharmacological rule that a drug is effectively cleared after four to five half-lives, most of a single dose has left the bloodstream within three to five days.
After an oral dose, ivermectin is absorbed reasonably quickly. Peak plasma concentration is reached at roughly four hours in the fasted state. A secondary peak often appears some hours later, which is attributed to enterohepatic recycling — the drug is excreted in bile, then partially reabsorbed from the intestine.
Taking the dose with a high-fat meal raises peak concentration and total exposure substantially, by around 2.5-fold in the manufacturer's pharmacokinetic work, without greatly changing the time to peak.
Ivermectin is highly lipophilic and roughly 93 percent bound to plasma proteins. It distributes widely into fat and into the liver, where concentrations stay well above plasma levels. Because of that fat reservoir, tissue persistence outlasts plasma persistence.
This is clinically useful rather than a concern: it is why a single dose can continue to suppress microfilariae for months in onchocerciasis, and why the drug retains activity in the skin long after plasma levels have fallen.
Crucially, ivermectin does not readily cross an intact blood-brain barrier. P-glycoprotein, an efflux transporter at the barrier, actively pumps it back out. This is the main reason the drug is far safer in humans than the same doses would be in animals lacking functional P-glycoprotein, such as some collie breeds with the MDR1 mutation.
Ivermectin is metabolised in the liver, principally by the CYP3A4 enzyme, into several hydroxylated and demethylated metabolites. This is the basis of most of its clinically relevant drug interactions — strong CYP3A4 inhibitors raise exposure, strong inducers lower it. See drug interactions.
Elimination is overwhelmingly faecal. Less than one percent of an administered dose is excreted unchanged in urine. Total elimination of drug and metabolites takes about 12 days.
| Time after a single dose | Approximate state |
|---|---|
| 4 hours | Peak plasma concentration |
| 18 hours | About half the dose eliminated from plasma |
| 36 hours | About 75% eliminated |
| 3–4 days | Plasma levels largely negligible |
| ~12 days | Drug and metabolites fully excreted |
| Weeks to months | Residual antiparasitic effect in tissue for some indications |
Liver function is the dominant variable. Because clearance is hepatic, impaired liver function prolongs exposure, and ivermectin should be used cautiously in significant hepatic disease.
Body composition matters too. A higher proportion of body fat increases the volume of distribution and can extend tissue persistence. Age, concurrent CYP3A4-active medicines, and whether the dose was taken with food all shift the numbers as well.
Ivermectin does not accumulate meaningfully at standard dosing intervals. The 7-to-14-day gap in a scabies course is long enough for near-complete plasma clearance between doses; the second dose targets newly hatched mites, not a top-up of drug level.
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View products and pricingRoughly 18 hours in plasma for most adults, with published estimates spanning about 12 to 36 hours. Metabolites persist longer, and complete excretion of drug plus metabolites takes around 12 days.
Plasma levels are negligible after three to five days. Complete elimination of the drug and its metabolites takes about 12 days. Antiparasitic activity in tissue can persist longer, which is why a single dose suppresses microfilariae for months in onchocerciasis.
Not at standard intervals. Doses spaced a week or more apart allow near-complete plasma clearance in between. Accumulation would only become a consideration with daily dosing over an extended period, which is not a standard regimen.
Plasma concentration peaks at about four hours, and the drug begins paralysing susceptible parasites within hours. Visible clinical improvement is slower and depends on the condition — days for intestinal worms, weeks for the itch of scabies to settle.
No. Standard workplace and clinical drug screens test for controlled substances and their metabolites. Ivermectin is not among them and is not detected by routine panels.