How is ivermectin dosing calculated from body weight using 3 mg tablets for common adult weights?
Executive summary
Ivermectin tablets for humans are labeled as 3 mg each and clinical dosing is calculated by body weight, typically using 150–200 micrograms per kilogram (mcg/kg) depending on the indication; the drug label and major references recommend weight-based single doses converted into milligrams and then into 3 mg tablets [1] [2] [3]. Practical prescribing uses the formula Dose (mg) = Weight (kg) × (mcg/kg) ÷ 1,000, then divides that milligram amount by 3 mg per tablet and rounds to an appropriate tablet count or to the manufacturer’s weight-bands [4] [1] [3].
1. How the math works: convert mcg/kg to tablets in three steps
The standard arithmetic is straightforward: choose the target microgram-per-kilogram regimen (for many parasitic infections guidance is 150 mcg/kg or 200 mcg/kg), multiply by the patient’s weight in kilograms to get micrograms, divide by 1,000 to convert to milligrams, then divide by 3 (mg per tablet) to get the tablet count; clinical references and the FDA label describe this weight-based approach [2] [1] [4].
2. Which mcg/kg to use depends on the infection and source
Different conditions use different targets: onchocerciasis is commonly dosed at about 150 mcg/kg while strongyloidiasis and many scabies protocols often use ~200 mcg/kg; guideline sources and the product label explicitly state the 150–200 mcg/kg range and indicate the selected mcg/kg varies by condition [1] [2] [5].
3. Examples for common adult weights using 200 mcg/kg and 150 mcg/kg
Using the formula above, a 60 kg adult at 200 mcg/kg: 60 × 200 = 12,000 mcg = 12 mg, which equals four 3 mg tablets [4] [6]; at 150 mcg/kg the same 60 kg person would get 9 mg ≈ three 3 mg tablets [1] [3]. For quick reference: 50 kg at 200 mcg/kg = 10 mg → ~3–4 tablets (3 tablets = 9 mg, 4 tablets = 12 mg); 70 kg at 200 mcg/kg = 14 mg → ~5 tablets (5 × 3 = 15 mg); 80 kg at 150 mcg/kg = 12 mg → 4 tablets per manufacturer weight bands; 90–100 kg at 200 mcg/kg give 18–20 mg → 6–7 tablets [4] [3] [7].
4. Manufacturers’ practical tablet bands and rounding rules
Because tablets are discrete, manufacturers and reference texts provide weight bands that map to whole tablets (for example: 1 tablet for 15–25 kg, 2 tablets for 26–44 kg, 3 tablets for 45–64 kg, 4 tablets for 65–84 kg, with ≥85 kg often handled by exact mcg/kg calculation) rather than insisting on fractional tablets; that banding is in prescribing documents and product summaries [7] [1] [3].
5. Caveats: absorption, repeated dosing, and medical supervision
Observed systemic exposure can vary with food and individual pharmacokinetics—taking ivermectin with a meal can increase absorption versus an empty stomach—so milligram dose is an approximation of exposure and not an exact body-delivered amount [8] [2]; some indications call for repeated or interval dosing (e.g., scabies regimens or mass-treatment studies with higher or repeated dosing), and official sources emphasize dosing should be determined by a clinician and monitored as recommended [2] [9].
6. Bottom line for clinicians and patients
Clinically, convert weight to kilograms, apply the indication-specific mcg/kg (commonly 150–200 mcg/kg), convert to mg and then to 3 mg tablets using manufacturer weight bands or rounding to whole tablets—always follow the product label and a healthcare professional’s instructions rather than self-calculating and self-medicating [1] [2] [7].