This is just a theory, but I think the reason the dewormers have great effect in humans is two-fold. One, I think a lot of humans in the United States have untreated/unrecognized parasites. And two, there is an anti-cancer mechanism of action of these dewormers.
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OK, before I get in trouble for diagnosing or treating humans, I don’t! I do not diagnose or treat humans! I’m a healthcare advisor for pets. But I commonly have people reach out to me, wanting help with novel cancer treatments in their pets. (These animals have already been diagnosed by their conventional veterinarian.)

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I’m not against using either of these medications necessarily.
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But let’s discuss some considerations:
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Consider the fact that these are incredibly commonly used animal dewormers. Fenbendazole is the very first dewormer a puppy gets. It has a couple of different names. Panacur and SafeGuard are common names. And it’s used month after month and year after a year in dogs. 
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Ivermectin is the basis to a very common heartworm preventative, Heartgard. 
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These medications have been around and used in pets so long that there are now many many reported cases of resistance. 
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So, while humans don’t commonly take these medications in this country, dogs take it like it’s candy. 
Which means, if dogs have parasites, the parasites are likely resistant to these medications.
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So, if parasites are at the basis for your pet’s cancer, or any other health condition, conventional western medicine dewormers may not be as useful in helping your pet. 
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Secondly, especially regarding ivermectin, humans take 100 times more of this medicine than the same size dog would take! It can be very toxic. So would I flat out advise against these? No. But do I think they are going to be the cure all that people are hoping for? No. Not that either.
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Absolutely,  I think part of helping a dog or a cat deal with cancer is determining if there are underlying parasites. Absolutely energetically, with muscle testing, that question can be asked. And the conventional medication can be included in the list of potential remedies.
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But how about the litany of herbal remedies that are excellent anti-parasitics as well? Artemisia, wormwood, black walnut tincture, Japanese knotweed, houttuynia, neem, noni. There are a myriad of great herbals that are anti-inflammatory in general, may be anti-cancer, and certainly may be anti-parasitic as well.
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And that’s simply looking at the parasite aspect that may be contributing to cancer. Because what is cancer? It’s inflammation gone wrong. Parasites can certainly play a roll in that. But, so can yeast/fungus, heavy metals, chemical and industrial, poisons and round up/glyphosate. If your pet is gonna have any chance of slowing down the effects of cancer, all of these poisons need to be gently removed from the body.
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In my experience, herbs, and diet, and my favorite thing in the whole world, raw goat milk, do an amazing job at detoxifying the body. 
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Note: this article is only talking about one potential underlying contributor to cancer – parasites. This article doesn’t talk about any of the other herbal anticancer treatments. Or diet. It’s 100% illogical to do a cancer treatment and yet allow your pet to continue eating the food that probably caused the cancer in the first place.
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But that’s a different post. 
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WARNING: As an aside, I’ve seen things not go well for pet owners, people  who are doing things out of 100% pure love! Neither of these stories below is a criticism, but it is  evidence that commonly use medications are not without side effect.
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I had one pet parent reach out to me after giving a portion of her human dose Ivermectin tablet to her labrador. He never walked again.

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Another pet owner gave a very conservative, very modified, low-dose amount of fenbendazole to her dog who had had an splenic tumor removed two weeks prior. He bled to death. After one dose.
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Still seeing patients in Tampa, Clearwater, and virtually.