Fenbendazole is a deworming drug used in many animals that is now under investigation as an anti-cancer drug.
- Fenbendazole treats parasitic worms in dogs. It may have potential as a cancer treatment as well.
- Side effects of fenbendazole are vomiting and diarrhea, but these are quite rare when it is given short-term to treat parasitic infections.
- You do not need a prescription for fenbendazole for dogs but should always discuss starting a new medication with your veterinarian.
- Fenbendazole may cause liver damage in dogs when given long term, as this has happened in a human.
- Ivermectin and fenbendazole are not the same medication, and work differently.
What Is Fenbendazole for Dogs?
Fenbendazole is a deworming/antiparasitic drug commonly given to dogs and other animals. It is part of a class of drugs called benzimidazoles.
It can be given as a pill, liquid, paste, or powder. The treatment duration can vary depending on what type of parasite is being treated.
Fenbendazole is not well absorbed in the intestine, with reports ranging from 10-50% absorption. This may be because it is not very soluble (easily dissolved) in water.
How Fenbendazole for Dogs Works
Fenbendazole acts by disrupting tubulin. Tubulin is a protein that is essential to produce the cellular structures called microtubules.
This is relevant to cancer therapy because microtubules are required for cell division and replication. The microtubules must assemble and disassemble into flexible lines for a cell to make a copy of itself – which cancer cells do a lot of!
In fact, many of our anti-cancer drugs, like vinculin, vincristine, Taxol, and paclitaxel all work by disrupting the process of breaking down and rebuilding microtubles.1,2
A Chance Discovery
Remember that fenbendazole was initially developed to kill parasites, not treat cancer. Microtubules of parasitic worms are very different from higher mammals like dogs, so it was not previously expected that fenbendazole would affect mammalian microtubules in the same way.
However, one lab stumbled on the possibility that fenbendazole might have anti-tumor effects in mice. This lab was studying lymphoma by injecting human lymphoma cells into mice.2 This was a well-established model to create cancer in lab mice but to their surprise the injections failed to produce tumors. The only difference they could identify was that the mice were being fed a diet with fenbendazole for an outbreak of pinworm (an intestinal parasite) in the facility.
The fenbendazole-laden diet was designed to be autoclaved (high heat sterilization) so it was supplemented with extra vitamins because high heat processing is known to degrade the nutritional content of food. But this facility did not have the equipment to autoclave the feed, so the diet was fed to the mice without autoclaving. That meant that in addition to getting a dose of fenbendazole, the mice also got a high dose of vitamins.
Fenbendazole Plus Vitamins
The lab decided to determine if the vitamins, the fenbendazole, or both together caused their tumor model to fail. They repeated the experiment and fed with 4 groups: one fed standard feed, one fed vitamin supplements, one fed fenbendazole, and one fed vitamins and fenbendazole.
This time around the tumor cell injections worked in all the treatment groups, but the researchers still made some interesting observations. The vitamins or fenbendazole alone did not affect the tumor growth compared to the control group. Fenbendazole had a trend toward increased tumor growth but not to a statistically significant degree.
But the mice that were fed fenbendazole and extra vitamins together showed a long delay before tumor growth began. These mice also had a higher level of neutrophils (a type of immune cell). The researchers concluded that fenbendazole with high dose vitamins may have inhibited tumor growth and proposed that other researchers consider this in their future tumor studies.
Research Ongoing, but So Much We Don’t Know
Since the study was completed in 2008, interest has arisen in fenbendazole’s effects on cancer, both in the interest of avoiding complications in future mouse studies and as a potential therapeutic in humans and other animals.
A few studies in cells grown in culture dishes have yielded some interesting findings and have begun to address how to best deliver the drug to cancer cells (remember that oral fenbendazole only has a 10-50% absorption rate).3-5 Some of these studies have included leukemia cells, prostate cancer cells, and canine melanoma cells.3-5
There was also a study in mice using a breast cancer model (not in combination with vitamins) which found no effect on tumors in the mice alone or in combination with radiation, but did see an effect on cells cultured in dishes.1
Overall, study of fenbendazole as an anti-cancer agent is in its very early stages and several aspects must be addressed, including:
- Improving delivery to cancer cells
- What cancers it might be effective in
- What therapies it might best be paired with
Be Skeptical of Incredible Claims
Internet claims suggest this drug can and should be used to treat every single cancer there is, which is usually a good cause for alarm.
After a human lung cancer patient claimed to be cured by fenbendazole and a bit of social media fervor in South Korea, the belief that this drug could cure non-small cell lung cancer took off (and led to at least one case of liver toxicity).6,7 A high profile singer and comedian in South Korea amplified the fenbendazole message when he was diagnosed with lung cancer, but later denounced his previous message after eight months of use, claiming no benefit and severe side effects. He later died of his disease.6
Fenbendazole Shows Promise, But Has Not Been Thoroughly Tested In Any Cancer
Fenbendazole has not been tested for efficacy in any cancer yet. The original tumor model that it affected was lymphoma, and hopefully research in that direction will be productive.
As it stands, this should be considered a promising potential therapy awaiting more testing.
Common Uses of Fenbendazole for Dogs
Fenbendazole is used routinely for short periods of time (days) to treat parasitic worms in dogs and other animals.
When to Not Use Fenbendazole
There are no known drug interactions with fenbendazole. However, it is mostly used in short courses of therapy, so we don’t yet know if there may be drug interactions that develop when it is used long term (such as for cancer treatment).
There was one known case of liver damage caused to a human patient taking fenbendazole in combination with her chemotherapeutic drugs without consulting her physician. As the drug is processed in the liver, it is possible that long term use with other drugs that stress the liver could be harmful.
Fenbendazole Dosage for Dogs
When used as a dewormer, this drug is usually given based on body weight. For example, Panacur recommends 50 mg/kg (22.7 mg/lb) per day.
Fenbendazole may come as a liquid, powder, or paste and is typically mixed with food. Treatment duration is based on what type of parasite is being treated.
What If I Miss a Dose?
This is a once daily medication so if you miss a dose and it is the same day give the dosage when you remember. Alter the dosage timing of the following doses to 24 hours from when you gave the last dosage.
For example, if you are giving the dosage every morning and you remember at dinner time, give the dosage at dinner and give the following doses that are due at dinner time as well.
If it has been more than 24 hours, then skip the missed dose and call your veterinarian to find out if you need to extend the course of treatment. Do not give a double dose.
Storage and Handling
Fenbendazole can be stored at room temperature (68-77 °F).
Safety and Side Effects
This is a well-tolerated drug when given over the typical time course, with the most common side effects being vomiting or diarrhea. These side effects are still quite rare, affecting usually about 1% of dogs. Some animals may hypersalivate after an oral dose which will resolve on its own.
We don’t yet know what side effects may occur when fenbendazole is given long-term, so there may be more interactions that show up as cancer studies continue.
Our Take on Fenbendazole for Dog Cancer
Although there is some promising research, at this point there is not enough evidence to recommend fenbendazole for cancer treatment.
We do not know enough about the long-term effects in dogs or interactions with chemotherapies. This is important to consider as its mechanism of action is similar to many chemotherapeutic drugs already used as first line treatment in cancer.
Fenbendazole may offer another line of treatment in the future. If you want to consider the use of this drug it is essential to discuss it with your veterinarian first.
- Duan Q, Liu Y, Booth CJ, Rockwell S. Use of fenbendazole-containing therapeutic diets for mice in experimental cancer therapy studies. J Am Assoc Lab Anim Sci. Mar 2012;51(2):224-30.
- Gao P, Dang CV, Watson J. Unexpected antitumorigenic effect of fenbendazole when combined with supplementary vitamins. Journal of the American Association for Laboratory Animal Science. 2008;47(6):37-40.
- Kim S, Perera SK, Choi SI, Rebhun RB, Seo KW. G2/M arrest and mitotic slippage induced by fenbendazole in canine melanoma cells. Vet Med Sci. May 2022;8(3):966-981. doi:10.1002/vms3.733
- Koohi Moftakhari Esfahani M, Alavi SE, Cabot PJ, Islam N, Izake EL. β-Lactoglobulin-Modified Mesoporous Silica Nanoparticles: A Promising Carrier for the Targeted Delivery of Fenbendazole into Prostate Cancer Cells. Pharmaceutics. Apr 18 2022;14(4)doi:10.3390/pharmaceutics14040884
- KalantarMotamedi Y, Ejeian F, Sabouhi F, et al. Transcriptional drug repositioning and cheminformatics approach for differentiation therapy of leukaemia cells. Sci Rep. Jun 15 2021;11(1):12537. doi:10.1038/s41598-021-91629-x
- Sultana T, Jan U, Lee H, Lee H, Lee JI. Exceptional Repositioning of Dog Dewormer: Fenbendazole Fever. Current Issues in Molecular Biology. 2022;44(10):4977-4986.
- Yamaguchi T, Shimizu J, Oya Y, Horio Y, Hida T. Drug-Induced Liver Injury in a Patient with Nonsmall Cell Lung Cancer after the Self-Administration of Fenbendazole Based on Social Media Information. Case Rep Oncol. May-Aug 2021;14(2):886-891. doi:10.1159/000516276
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