How can fleas be simultaneously 100% natural and bad?

Fleas are cool, but they suck

Fleas are extraordinary organisms. Since I’m going to spend the rest of this post discussing ways to kill them (don’t worry flea enthusiasts, many of the following methods don’t actually work!), I thought it would be fair to first acknowledge some of the amazing things they do. Were you aware, for example, that flea larvae eat their parents’ feces and cannibalize their siblings’ eggs? Or that they can jump 100 times their body length? Did you know they have a pad of almost perfectly elastic material in their leg that shoots them into the air at about eight miles an hour? When we’re thinking about fleas on dogs, we’re actually thinking of the cat flea, Ctenocephalides felis, which is the most common external parasite of dogs and cats in the world. Did you know you can identify different species of fleas by their mustaches (or genal combs)? Here’s a cat flea, here’s a dog flea, here’s some mouse fleas, here’s a chili flea. Fleas as a group co-evolved with mammals and have been around since the Cretaceous Age.

All right, enough flea fluffing, let’s talk about killing these suckers. We should kill fleas because a lot of our pets are extremely allergic to flea bites and it makes them miserable. They are also vectors for zoonotic disease such as rickettsial diseases, cat scratch disease, plague, and tapeworms. As individuals, fleas are not that hard to kill. A well-placed fingernail will generally work. The problem is that they reproduce rapidly and persist in the environment for a long time. Their life cycle is extremely fast. One flea can lay 50 eggs a day for 2-3 months and there may be around 15 fleas on your average infested cat. And they can complete their entire life cycle, from thousands of tiny eggs (which live in your floors and carpets) to hundreds of bloodthirsty adults, in three weeks. You don’t need to be too good at math to know that if you keep multiplying numbers like this, you’ll run out of space on your Casio fx-85v calculator pretty fast. Unless you’ve gotten to level 384 in Galaga, you’re not going to be able to kill these insects individually. You need control at a population level.

This is the fundamental problem with fleas. Because their life cycle is so aggressive, it’s not enough to do a one-time treatment and expect the problem to be fixed. If you have any gaps in coverage, even a few weeks, the infestation can become re-established. You don’t want to give them an opportunity to bite, feed, f—, and lay eggs. Young flea stages can stay alive for months waiting for a host, so there may be a potential reservoir already in the house looking to emerge. That makes it critical for effective control to provide around-the-clock flea killing. I can’t overstate this point. If you want to get rid of the fleas, you need a scorched earth policy on their entire population. If you don’t, you’re leaving the door wide open.

The good news is that there are some very effective ways to do this. We have molecules that kill fleas quickly, effectively, provide long-lasting coverage, and are generally extremely safe for pets. These molecules typically work by selective toxicity, which means they are much more toxic to some species (i.e. fleas) than to others (i.e. dogs and cats). They take advantage of some fundamental differences in physiology between insects and mammals, which are usually pretty dissimilar at a cellular level. The more specific the difference, the more selective the toxicity, which means that the drug both works better to kill the pest, and is safer to use on the host species.

There are different categories of commercially available flea products, and guidelines on how to evaluate them. The two most important things to consider are efficacy (which is a measure of how well the compound kills the fleas in a realistic environment), and its safety to the animals we are using it on. In general, veterinarian-prescribed flea medications have good studies showing that they are safe and effective. They go through multiple trial phases, from in vitro studies to show something might work, to real-world field trials with people’s actual pets. There are other types of products that you can buy over-the-counter, most of which have efficacy and safety data. This is how the evil companies that produce these useful products got approval from the FDA or the EPA to market their product.

There’s one more category of flea products available to the consumer, however, and it’s the type that has neither safety nor efficacy evidence: the “alternative” flea treatments. Because there is a general misunderstanding that synthetically produced chemicals are always bad, and naturally produced chemicals are always good, a market has sprung up for these alternative therapies. If you visit any natural pet health website or store, you will likely come across a lot of products that claim to kill fleas without using so-called “harsh” chemicals. You will be told that these natural, organic, “chemical-free” products are safe, work well, and are safer for your beloved pet than the products made by greedy pharmaceutical companies. They don’t ever have any studies to support them, but they do sound a lot nicer. There’s no reason they can’t be studied, and they are already being sold en masse, so you’d figure the people advocating their use would test them out, but not yet. Because these alternatives don’t have any data to support their use, I like to call them Scientifically Testable but Unproven Pesticides, Insecticides and Devices; or STUPID for short.

Yup, I used that word. Pesticide. It’s a word that just means “kills bugs“. If you want to kill fleas you have to use a pesticide. It can synthetic or organic, but if it kills bugs, it’s a pesticide. You may not want to use a pesticide, fine! Nobody’s forcing you to kill fleas. But assuming your natural flea product is designed to kill fleas, then it’s a pesticide. There is nothing fundamentally different about a naturally-derived chemical that kills fleas and a synthetically engineered one, except for marketing. For example, if I give you the name of a compound, can you guess if it is a “harsh” chemical or a safe natural alternative?

Let’s try “allicin”.

How about “spinosad”?

Both of these are chemicals, and both of them are organic molecules. Allicin is a natural compound found in fresh garlic that has been proposed as a natural flea treatment but has no efficacy data. Spinosad is a natural insecticide from bacteria that has been artificially synthesized for parasite control and is supported by good evidence and safety. Most natural pet health websites will adamantly recommend garlic for fleas but tell you to avoid a product that contains spinosad. Does that make sense? It’s a classic appeal to nature fallacy.

Where I finally stop rambling and get to the point

That was a long rant! Thanks for sticking with me, and my apologies for the detour. But background information is important, since understanding flea biology is so crucial to why we use the products we do. I promise now to actually talk about alternative flea control methods and the evidence in support of them, but (spoiler alert) there’s not a lot. I tried to be charitable and explain proposed mechanisms when clinical trials were lacking. The only reasonably good trials we have show that alternative therapies don’t work. But anyway, here’s a list of what is currently being advertised to chemically-conscientious pet owners.


This one has to be the most commonly-recommended natural flea repellant. The garlic plant contains alliin, which is the precursor for all of the supposedly effective compounds. There are thousands of studies on garlic molecules, as they have in vitro antimicrobial, antifungal, and antiprotozoal properties. Napalm also has antimicrobial properties, but I can’t recommend it. There’s one trial on the use of garlic for flea control in dogs; there were only three dogs in it, no control group, and it appears a pile of fresh garlic was just dumped on the ground next to dogs in a confined space. I’m trying to be generous but it ironically doesn’t pass the smell test. Also, consider that in high enough volumes garlic has antidog and anticat properties. Thank you Skeptvet for collating a bunch of other stinky garlic health claims.

Diatomaceous earth

Another pretty common recommendation is diatomaceous earth, which is the fossilized skeletons of microscopic organisms. Leaving out whether or not it’s disrespectful to use the bodies of these Miocene organisms to murder Holocene arthropods, diatomaceous earth is a pesticide and kills insects when applied topically. It works by absorbing oil from the parasite and causing death by desiccation. The best evidence I could find for the use of diatomaceous earth was a trial in chickens which showed parasite reduction but not elimination. Tapatio hot sauce might also be effective in reducing parasite load, but it would not expected to break the life cycle or eliminate an infestation. Diatomaceous earth can also be irritating when inhaled. Personally I find most things other than air are irritating when inhaled, and since you’d need to use this on almost a weekly basis to provide good flea control, you’re better off avoiding it.

Essential oils

You knew EOs would make an appearance here! There are way too many different types of essential oils and possible uses to go into but a few different ones are recommended for flea control. Some can kill and/or repel insects, but there is only a single in vitro study of one essential oil showing efficacy against fleas. No clinical trials. With no information as to safety, duration, and practicality, this is so far from being clinically useful it might as well be a flea-killing nanobot. It is worth noting that some essential oils are directly toxic to dogs and cats.

Apple cider vinegar

The darling of natural healers looking to get pH back out of whack. There is exactly zero evidence that this is effective, and no plausibility. The vinegar part is acetic acid, which is a well-known antiseptic. The apples part sound healthy and the cider part sounds delicious. Acetic acid is great for cleaning counters but it does not selectively kill blood-sucking insects. I know this because cattle and other ruminants basically run off of acetic acid and other volatile fatty acids (the reason they have big bellies is that’s where the microorganisms ferment grass into those fatty acids), and their blood does not kill insect pests. I’m so happy I took ruminant physiology in vet school.


Most natural pet websites recommend adding beneficial nematodes to your yard because there are some nematodes (and fungi) that kill fleas. But since the flea infestation is usually in the house and these worms can’t live in your carpet, it would be pretty unlikely that they would make a practical difference. You know who else eats fleas? Cats that are grooming themselves, but nobody recommends adding them to your potting soil.


There are so many other unsupported claims I really can’t go into all of them: coconut oil, Baltic amber resin, raw diets, special plants in your garden, et cetera. It goes on and on, always claiming to be perfectly safe and always more or less guaranteed to work. When we do have studies on alternative therapies, they’re negative. Special collars purporting to create a bio-resonance field didn’t work, but are still being sold. Ultrasound collars didn’t work either, but are also widely available for sale. Aloe juice powder didn’t work. Brewer’s yeast didn’t work. If you’re looking for published science on any other natural or alternative flea control, that’s where it ends.

The bar is relatively low at “show a shred of in vitro efficacy”, and it’s not hard to kill any one flea, especially in a controlled environment. In fact, this might be something that acupuncture works for! But demonstrating real world benefit, specifically that all-important, long-lasting effect for population control, is something that all of these other methods consistently fail at. That never stops a new product from popping up, as there’s an endless line of people willing to manipulate pet owners fear for profit.

Is there any reason to not use flea medications?

The question is then: if we have lots of products that are proven to work, and lots of alternatives that are not proven to work, why would anyone choose the latter? My guess is fear, so let’s investigate the risks of the standard flea treatments. Is there any evidence that those products cause chronic disease, cancer, or other negative health consequences for the vast majority of pets that use them? No. I don’t think a reasonable person would argue that there are zero risks to these products, but they generally need to have extensive safety research (something most if not all of the alternatives lack) to get government approval. If they are not used correctly, they can be toxic. One of the most common problems is when an owner applies a permethrin-containing flea product for dogs to a cat. Cats lack an enzyme that metabolizes the permethrin, making them really sensitive to this class of drugs.

Is there a risk to human health? It appears pretty minimal, as long as they are appropriately used. That is a big caveat, because humans can always find ingenious ways to screw things up. But remember, we’re using the compounds precisely because they don’t affect us hair-growing, homeothermic mammals. You can’t necessarily say the same thing about some of the alternative pesticides, which in some cases may be more toxic to people. You can always look up information for any specific product on the FDA website or on the National Pesticide Information Center website.

Finally, is there a risk to the environment? Some of the drugs used are potentially environmental hazards. Native invertebrate species (as well as fish and birds) can be negatively impacted by pesticides, however, it is much more likely that agricultural use is responsible for the damage, rather than flea protection for pets (see here, here, here, and here). Using an oral flea product and cleaning up your pet’s waste would conceivably reduce the environmental risk. This is also why getting more and more specific with our pesticides is good, since it will limit damage to off-target species that we care about.

Conclusion: Don’t fall for unproven junk, you already have great flea control options

What do you want?! To have a pesticide-free cake and not have parasites eat it too? If you want to kill pests but think pesticides are the wrong way to go about it, you’re worried about the wrong things. If you think dogs and cats get fleas because they’re eating processed foods and getting vaccines, you have forgotten about the 1800s when we had to hire monkeys to pick fleas off of our pets. If you are suspicious of all products made by German-sounding giant multinational corporations, don’t be surprised if you can’t get rid of your flea problem. I understand that big Pharma has deeper pockets than Big Garlic, but I’m sure we could put some of these alternative therapies to the test. Just don’t trust advice from people who are trying to scare you, and often selling you an unproven alternative. Demand some evidence from these people. It would be great if we had a cheap, readily available, completely safe, environmentally risk-free, perfectly efficacious way to keep fleas off of our animals. As it stands, we’re pretty close with the modern flea preventatives.

One last thing: someone (perhaps many someones) is going to jump in and tell us an anecdote about how they used Dr. Bronner’s soap or something for fleas and it worked! I don’t doubt their experience. But please, try to understand that your observation of something does not prove anything. Fleas are a common problem, but there is a lot of variation in how much they affect any one animal, and exposure and risk are not the same everywhere at all times. If your experience is true, it should be provable. It really would not be that hard to prove these alternative therapies work, since all you need is fleas and dogs, which are abundant. Oh wait, you’d need science too, and that might mess things up.



  • Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.

Posted by Greg Bishop

Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.