How is ivermectin dosing calculated by weight and what are standard regimens for common parasitic infections?
Executive summary
Ivermectin dosing is weight‑based, typically calculated in micrograms per kilogram and most often given as a single oral dose; common regimens for human parasitic infections fall between about 150 and 400 μg/kg depending on the disease and programmatic goals (150–200 μg/kg for onchocerciasis and strongyloidiasis; up to 400 μg/kg used for lymphatic filariasis) [1][2][3]. Tablets are manufactured in fixed milligram strengths (commonly 3 mg) so prescribers convert the patient’s weight to the target mcg/kg dose and then round to whole tablets per an approved dosing table or label guidance [4][5].
1. How dosing is calculated in practice: the math and the pills
Clinicians calculate ivermectin dose by multiplying the patient’s weight (in kg) by the target micrograms per kilogram (mcg/kg) and then converting micrograms to milligrams (1,000 mcg = 1 mg) to determine how many tablets of, for example, 3 mg each are needed; manufacturers and labels provide dosing tables to translate weight into whole tablets [4][5]. The FDA label for human stromectol was designed to provide approximately 200 mcg/kg and instructs taking tablets on an empty stomach with water, and in most indications a single dose is sufficient though follow‑up testing is recommended [6][3].
2. Standard regimens for intestinal strongyloidiasis and onchocerciasis
For nondisseminated Strongyloides stercoralis, the standard human regimen is a single oral dose of approximately 200 μg/kg, with follow‑up stool or concentration techniques to verify eradication [7][6][3]. For onchocerciasis (river blindness) mass‑drug administrations and clinical use most commonly employ single doses around 150–200 μg/kg, often repeated at intervals (for example annually) because ivermectin clears microfilariae but does not reliably kill adult worms [1][8][3].
3. Higher and disease‑specific dosing: filariasis and special programs
Certain programs and infections call for higher ivermectin exposure: lymphatic filariasis control has used regimens approaching 400 μg/kg, and research trials have explored even higher or repeated dosing for novel public‑health aims such as reducing malaria transmission or vector control, though such uses are investigational and increase the risk of dose‑related adverse events [1][8]. Systematic reviews and program experience indicate doses from 200–400 μg/kg are generally considered safe when tailored to the pathology, but higher dosing has been linked to ocular and other adverse events in some settings, so risk–benefit and local endemic co‑infections (for example Loa loa) must be considered [1][9].
4. Regimens for ectoparasites like scabies and lice
Off‑label oral ivermectin is widely used for scabies: dosing is commonly 200 μg/kg given in two doses 7–14 days apart for classic scabies, while crusted scabies regimens involve multiple doses spaced over days and weeks depending on severity; for head lice and other ectoparasites oral dosing is also weight‑based but often repeated as clinically indicated [10][11][12]. Dermatology sources emphasize ivermectin’s favorable safety profile and anti‑inflammatory effects, but note oral formulations are adjuncts and sometimes used alongside topical therapy [12][11].
5. Safety caveats, exclusions and monitoring
The product information and clinical references stress that ivermectin is approved for particular parasitic infections and is not approved for unrelated uses such as COVID‑19; misuse and dosing from veterinary formulations are dangerous [13][7]. Adverse events range from common, mild reactions to rare neurotoxicity and interactions via P‑glycoprotein pathways; clinicians are advised to screen for co‑infections like Loa loa in travelers from endemic areas and to perform follow‑up testing where indicated [9][13].
6. Where guidance diverges and where ongoing research is focused
Guidelines converge on weight‑based dosing in the 150–200 μg/kg range for the classic indications, but published reviews, program policy papers and dose‑finding trials document variation—higher doses for filariasis, repeated dosing schedules for scabies or onchocerciasis programs, and research into mass‑drug administration or vector‑control roles—so prescribers follow indication‑specific protocols and regulatory labels rather than ad hoc regimens [1][8][2].