What adverse effects did the 2021 ivermectin oncology trials report?
Executive summary
Clinical reports of ivermectin used in human trials in 2021 do not describe a dedicated oncology trial reporting adverse effects; most 2021 clinical work cited involves COVID-19 trials where reported adverse events were monitored but showed no clear safety signal in high‑quality trials (e.g., NEJM and JAMA trial reports) [1] [2]. Preclinical 2021 oncology studies (mouse and cell work) reported antitumor activity but not human adverse‑event profiles; commentators and later phase‑1 abstracts note only limited, mostly mild expected adverse effects in humans and caution about neurological toxicity at high doses [3] [4] [5] [6].
1. What the 2021 “oncology” literature actually was
The landmark 2021 items often cited about ivermectin and cancer are preclinical laboratory papers showing that ivermectin can induce immunogenic cell death in breast‑cancer models and convert “cold” tumors to “hot” in mice; these are mechanistic and animal studies and do not report human adverse events [3] [7]. Available sources do not mention a 2021 randomized human oncology trial that reported adverse effects; clinical human safety data discussed in the provided material come mainly from non‑oncology (COVID‑19) trials or later small phase I/II oncology abstracts [3] [1] [6].
2. Safety data from 2021 human trials (largely COVID‑19) — what was reported
High‑quality 2021 clinical trials of ivermectin for COVID‑19 monitored and reported adverse events, with the NEJM report describing reportable events including serious adverse events and discontinuations and that the data and safety monitoring committee met repeatedly while conducting interim safety reviews [1]. JAMA’s April 2021 randomized trial also tracked adverse events in a placebo‑controlled design [2]. These COVID trials generally did not flag a new, large safety signal at typical human dosing in their published reports, but they were not oncology studies and their populations and dosing differ from any proposed oncologic regimens [1] [2].
3. What later oncology-phase reports and reviews say about human adverse effects
A phase I/II abstract presented in 2025 of ivermectin plus the PD‑1 antibody balstilimab in metastatic triple‑negative breast cancer reported that dose levels 1 and 2 were completed with only one serious adverse event attributed to disease‑related anemia and concluded the combination was “safe and well tolerated” in that small cohort — but this is a 2025 abstract, not a 2021 trial report [6]. Reviews and trade pieces emphasize that human clinical evidence in oncology remains scarce and that most safety observations come from case reports, self‑medication harms, or non‑oncology studies [8] [5].
4. Known and potential adverse effects cited across the reporting
Sources consistently list mild, common adverse effects (gastrointestinal upset, skin irritation) at approved antiparasitic doses and flag that high or veterinary doses can cause serious neurological problems — confusion, disorientation, muscle issues, delirium and even coma in poisoning cases — usually described in the context of misuse during the COVID‑19 surge in 2021 [4] [5] [7] [9]. Systematic reviews of ivermectin trials for COVID‑19 noted low‑quality evidence around adverse‑event summaries, meaning safety conclusions there are limited by study quality [10] [11].
5. Two competing perspectives in the sources
One perspective (laboratory researchers and preclinical advocates) stresses promising mechanisms and mouse efficacy that justify human trials; those studies do not and cannot report human toxicity [3] [7]. The countervailing perspective, voiced by oncologists and evidence‑reviewers, warns that doses effective in mice may be toxic in humans and that there is currently no robust human oncology evidence of benefit, so risk–benefit is unknown and patients self‑medicating risk harm [5] [12] [8].
6. What is missing or uncertain in the reporting
Available sources do not mention a 2021 human oncology trial that reported a formal adverse‑event table for cancer patients; the human safety data in the record come primarily from COVID‑19 trials or later small oncology phase I/II work [1] [2] [6]. There is limited high‑quality human safety data specific to oncology dosing, combinations with immunotherapy, or long‑term effects; systematic reviewers repeatedly call human oncology evidence “scarce” [8] [11].
7. What patients and clinicians should take away
Ivermectin’s 2021 oncology signal is preclinical; it did not produce a human oncology adverse‑event profile in published 2021 reports because human oncology trials were not reported then. Human trials from 2021 focused on COVID‑19 tracked adverse events and did not show a distinct safety catastrophe at typical doses but cannot be extrapolated to potential oncology regimens; clinicians warn against off‑label self‑use because high or veterinary doses have caused serious neurological toxicity [1] [2] [5] [9].