Fenbendazole for dogs is a widely used and safe anthelmintic that effectively treats intestinal parasites. Unlike most wormers that only destroy adult worms, fenbendazole kills the entire population of intestinal parasites, including eggs and larvae. This prevents the recurrence of intestinal parasite infections, such as roundworms and hookworms, in addition to protecting against flukes and giardia in dogs. It is also used off-label to protect against lungworms in puppies.
Unfortunately, despite the widespread use of this drug and its safety, a number of unlicensed veterinarians have made videos on social media platforms, such as TikTok, that claim that fenbendazole cures cancer. While there are similarities between parasitic cells and cancer cells that suggest that anthelmintics might have anti-cancer properties, no peer-reviewed studies have found that animal anthelmintics such as fenbendazole cures canine or human cancer.
During chemotherapy, doctors inject the patient with powerful drugs that flood the body’s bloodstream with poisonous toxins that destroy all fast-growing cells—including healthy cells such as blood cells. These chemicals are effective at killing cancer cells, but they often damage healthy tissue as well, leading to severe side effects. This is why it is important to minimize the amount of chemotherapy given and to limit its duration.
As a potential alternative to chemotherapy, some scientists have sought out naturally occurring compounds that are thought to have anti-cancer properties. One group of such agents is called nitroheterocyclic chemotherapeutics, which have been shown to inhibit the growth of some types of cancer cells. Other groups of nitroheterocyclics are thought to act as radiosensitizers, enhancing the effectiveness of radiation to kill cancer cells.
To determine whether fenbendazole might have radiosensitizing properties, we treated EMT6 carcinoma cells with different concentrations of fenbendazole prior to and during radiation. Cells were treated with fenbendazole dissolved in sterile, pyrogen-free physiologic saline and injected i.p. Fenbendazole was not toxic to aerobic EMT6 cells at doses up to the limit of solubility. However, a 24-h treatment with fenbendazole significantly decreased the yield-corrected survival fraction of cells following radiation.
The effect of irradiation on tumor cells treated with different concentrations of fenbendazole and the related compound albendazole was similar to that observed for EMT6 cancer cells exposed to radiation alone. Both fenbendazole and albendazole significantly reduced the survival of irradiated tumors, but the effect was more dramatic for albendazole than for fenbendazole.
In experiments designed to mimic the cellular response to radiation that is seen in vivo, mice with tumors were treated with three daily i.p. injections of fenbendazole or vehicle, followed by the administration of 10 Gy of ionizing radiation. Tumors were evaluated for tumor growth after treatment, and the results were compared to those of control tumors. The growth curves of irradiated tumors that had been treated with fenbendazole or vehicle were identical to those of untreated controls. In contrast, the growth of irradiated tumors that received fenbendazole before and during radiation was inhibited. This result suggests that fenbendazole and albendazole have radiosensitizing activity and may be useful in combination therapy for treating cancer.