Peer-Reviewed by Dr. Marcus Vance, MD, PhD
Lead Clinical Pharmacologist, Institute for Clinical Pharmacology & Safety — Board-certified in Clinical Pharmacology & Toxicology. Reviewed for scientific accuracy against current FDA label documentation, WHO technical monographs, and peer-reviewed literature.
The 3 mg Tablet: Standard Unit of Clinical Dosing
Stromectol is commercially available in 3 mg oral tablets. This is not an arbitrary dose unit—it reflects the weight-based dosing paradigm underpinning essentially all ivermectin therapy in humans. Prescribers do not administer a flat dose; they calculate a dose per kilogram of body weight and then round to the nearest available tablet count. Understanding this relationship is fundamental to safe clinical use.
The 3 mg tablet provides a convenient denominator. A patient weighing 60 kg receiving a standard 200 mcg/kg dose would receive approximately 12 mg, or four tablets. Practical rounding typically occurs within a ±25 mcg/kg band without meaningful impact on therapeutic outcome or safety margin—though practitioners should consult product labeling for precise guidance rather than relying on approximations alone.
Weight-Based Dosing: Approved Indications
Intestinal Strongyloidiasis
The FDA-approved dose for strongyloidiasis caused by Strongyloides stercoralis is 200 mcg/kg as a single oral dose. This treats the adult intestinal worm burden. Follow-up stool examinations are typically recommended at three months post-treatment to confirm parasitological cure, as the drug does not penetrate well into larvae in the autoinfection cycle under all conditions. In immunocompromised patients with hyperinfection syndrome, treatment approaches become more complex and require specialist input.
Onchocerciasis
For onchocerciasis (Onchocerca volvulus), the approved regimen is also 150 mcg/kg as a single oral dose, administered every 6 to 12 months. The drug targets microfilariae (the larval stage) rather than adult worms; it does not kill adult Onchocerca worms outright, which is why repeated annual doses are necessary within endemic control programs. This is not a pharmacological limitation to be criticized—it is simply the biological reality of a long-lived, ensheathed nematode that presents a difficult drug-penetration target.
Dosing Table — Stromectol 3 mg Tablets
| Body Weight (kg) | Strongyloidiasis (200 mcg/kg) | Onchocerciasis (150 mcg/kg) | Tablets Required |
|---|---|---|---|
| 15–24 kg | 3–4.8 mg | 2.25–3.6 mg | 1 tablet |
| 25–35 kg | 5–7 mg | 3.75–5.25 mg | 2 tablets |
| 36–50 kg | 7.2–10 mg | 5.4–7.5 mg | 3 tablets |
| 51–65 kg | 10.2–13 mg | 7.65–9.75 mg | 3–4 tablets |
| 66–79 kg | 13.2–15.8 mg | 9.9–11.85 mg | 4–5 tablets |
| 80–100 kg | 16–20 mg | 12–15 mg | 5–6 tablets |
| >100 kg | Consult prescriber | Consult prescriber | Variable |
Reference dosing only. Actual prescription must be determined by a licensed healthcare professional based on current labeling and patient-specific factors.
Pharmacokinetic Parameters in Detail
The pharmacokinetic profile of ivermectin in human subjects has been characterized through multiple single-dose and repeat-dose studies in both healthy volunteers and patient populations. The core parameters relevant to clinical dosing decisions are discussed below.
Absorption and Food Effect
Oral ivermectin is absorbed from the proximal gastrointestinal tract. Time to peak plasma concentration (Tmax) is approximately 4 hours under fasted conditions. This is clinically important: co-administration with a high-fat meal substantially increases bioavailability. One pharmacokinetic crossover study demonstrated that a standard high-fat breakfast increased ivermectin AUC by roughly 2.6-fold and Cmax by approximately 2.5-fold. Some prescribers leverage this interaction deliberately for indications requiring higher systemic exposure; others administer fasting doses for consistency. Neither approach is universally standardized across all indications.
Half-Life and Plasma Clearance
The terminal elimination half-life (t½) of ivermectin in plasma is reported as approximately 12 to 36 hours across the adult literature, with a commonly cited central estimate near 18 hours in healthy adults. This range reflects genuine pharmacokinetic variability—differences in body composition (ivermectin is highly lipophilic and accumulates in adipose tissue), hepatic enzyme activity, and the food-effect interaction all contribute to inter-individual variation. Elderly patients and those with hepatic impairment may exhibit prolonged half-lives.
The practical implication: for single-dose protocols, most of the pharmacologically active drug has cleared within 72 to 96 hours in typical adults. Tissue-associated drug may persist longer, which has mechanistic relevance for the prolonged activity observed in some parasite-killing studies but carries limited clinical relevance for routine single-dose applications.
Volume of Distribution
The apparent volume of distribution is large—estimates range from approximately 40 to 50 L/kg—consistent with extensive tissue partitioning. The drug concentrates in adipose tissue, liver, and adrenal glands. This lipophilicity underpins both its wide tissue distribution and its relatively slow terminal elimination.
Protein Binding and Metabolic Pathway
Approximately 93% of circulating ivermectin is protein-bound, primarily to albumin. This high protein binding limits the free fraction available for CNS penetration and renal filtration, and contributes to the low urinary excretion observed (less than 1% of administered dose). Hepatic metabolism via CYP3A4 dominates, producing multiple hydroxylated and demethylated metabolites. Drug interactions with CYP3A4 inhibitors or inducers are theoretically significant but rarely reported as clinically problematic at standard single doses. Patients on concurrent strong CYP3A4 inhibitors (e.g., ketoconazole, certain HIV protease inhibitors) warrant prescriber awareness.
Special Populations
Pediatric patients: Safety and efficacy data exist for children weighing 15 kg and above. Weight-based dosing applies equally. Children under 15 kg have insufficient safety data in the current labeling.
Renal impairment: Given that less than 1% of drug is excreted renally, dose adjustment for renal impairment is generally not required. This represents a practical advantage in parasitically exposed populations where renal comorbidities may co-exist.
Hepatic impairment: Cautious use is advised. No specific dose adjustment recommendations are codified in the current label, but clinicians should be alert to prolonged half-life and reduced metabolic clearance in patients with significant hepatic dysfunction.