What is the half life of moxidectin
Executive summary
Moxidectin’s elimination half-life varies by species, formulation and route: in humans the terminal half-life is reported around 20–35 days (mean 20.2–35.1 days or 23.3 days after an 8 mg dose) [1] [2]. Animal data show much wider variation — from hours in some sheep studies to weeks in dogs and other species — reflecting dose, route (oral, topical, injectable) and tissue distribution [3] [4] [5].
1. What “half‑life” the literature actually reports
Pharmacokinetic papers and product labels most commonly quote the terminal (elimination) half‑life, which reflects the slow phase when drug leaves deep tissue stores, not the initial distribution drop. Human studies and the FDA label report a terminal half‑life of roughly 20–35 days — for example the FDA label gives a mean terminal half‑life of 23.3 days (559 hours) after a single 8 mg dose [2] and clinical PK summaries report means of 20.2–35.1 days [1].
2. Why values differ across reports
Different studies sample at different times and use different metrics (terminal vs distribution phase), produce different half‑life estimates. A clinical PK study noted a pronounced distribution phase in the first 24 hours followed by a prolonged elimination phase of about 18–24 days [6]. Older or small studies sometimes report shorter means (for example 11.5 days in an early tolerability study), reflecting limited sampling or different populations [7].
3. Big differences across species and formulations
Animal pharmacokinetics diverge sharply. Some sheep data reported elimination half‑life measured in hours (about 19.5 hours for total 14C in one study) [3], whereas oral or topical use in dogs and other mammals shows much longer terminal phases — a beagle study reported ~621 h (≈25.9 days) mean terminal half‑life and topical canine products show a terminal half‑life around 342 hours (~14 days) in some reports [4] [5]. Scientific reviews note typical elimination half‑lives in many species around 19 days or longer owing to lipophilic tissue storage [8] [9].
4. Mechanism behind the long residence time
Moxidectin is highly lipophilic and distributes into adipose tissue, which acts as a reservoir releasing drug slowly and producing a long terminal half‑life; it is also a poor substrate for P‑glycoprotein which reduces elimination from tissues and parasites, further prolonging residence time [8] [10]. Clinical pharmacology papers explicitly connect large volume of distribution and low oral clearance to mean terminal T1/2 values of about 17.7–23.3 days in onchocerciasis patients [6].
5. Human clinical range and product labeling
Regulatory and clinical sources converge on an elimination half‑life in humans on the order of weeks: clinical PK summaries put the mean elimination half‑life between ~20.2 and 35.1 days [1], and the FDA label specifies 23.3 days (559 hours) after a single 8 mg dose [2]. Reviews of moxidectin as an oral human therapy cite typical human half‑life ranges of ~20–43 days in broader literature summaries [10].
6. Conflicting or outlying numbers and why they appear
Some early or small human studies reported shorter means (e.g., 11.5 days) and some animal studies report half‑lives in hours; these are not errors but reflect methodologic differences (sampling duration, analytic methods), different definitions (short distribution half‑life vs terminal half‑life), and species‑specific ADME (absorption, distribution, metabolism, excretion) [7] [3]. Reviews warn that sampling schedules that stop early will underestimate terminal half‑life [6].
7. Practical implications for dosing and safety
A long terminal half‑life in humans implies prolonged exposure after a single dose, which can increase duration of effect against parasites but also means drug persists in milk and tissues and may interact with co‑infections (e.g., Loa loa risk screening is recommended before treatment) [2] [11]. Veterinary products exploit long tissue residency for extended protection; formulations (microspheres, topical) further alter apparent half‑life and duration of activity [5].
8. Bottom line for the original question
If you mean humans and the terminal (elimination) half‑life, current clinical and regulatory sources consistently report roughly 20–35 days (commonly cited ~23 days after an 8 mg oral dose) [1] [2]. If you mean animals or other formulations, half‑life varies widely — from under a day in some tissue‑total sheep measures to several weeks in dogs and other species — so specify species and route for a precise number [3] [4] [5].
Limitations: available sources differ by species, formulation, sampling method and date; I relied only on the provided sources and did not search beyond them.