There’s no evidence that vinegar kills or repels fleas. Some webpages and books mention using vinegar, usually apple cider vinegar, for flea control, but these claims are unfounded and anecdotal. The only references in scientific literature are outdated, written hundreds to thousands of years ago.
Vinegar as an Insecticide
Vinegar is sometimes recommended online as a natural way to combat fleas. The acetic acid is said to have insecticidal properties. However, aside from uncited sources and anecdotal claims, there’s no evidence that vinegar works as an insecticide.
Few pieces of scientific literature mention using vinegar to kill insect pests. Those that do were written hundreds of years ago. A document from 1775 contains a couple of vinegar insecticide recipes. The remedies involve mixing vinegar with unconventional ingredients, such as ox gall (bile) or strong glue. The concoctions were then to be rubbed onto the cracks and joints of bed frames, which supposedly killed all bugs within a short time.
Vinegar as an Insect Repellent
Uncited sources also recommend using vinegar as a natural flea repellent. Some claim that bathing pets with a vinegar solution will keep fleas away. Others state that adding a small amount of vinegar to their pet’s drinking water will repel fleas. The underlying assumption is that these methods alter the pH of animal’s skin and blood, making it too acidic and thus unpalatable for fleas.
There’s little evidence that vinegar repels insects, including fleas. Again, the sources are outdated. A 10th century remedy recommends rubbing a mixture of vinegar, manna, and oil on the body to repel gnats. Natural vinegars may have a slight repellent effect, namely against mosquitoes. The acids could have a mild antibacterial effect on the skin, therefore reducing the production of bacterial metabolites which mosquitoes use to locate hosts. Conversely, apple cider vinegar is often used as an attractant for some insect pests, such as vinegar flies and fruit flies. Pyroligneous acid, or wood vinegar, shows some more promising insect repellent properties.
While vinegar’s repellent properties are largely unfounded, it will alter the pH of an animal’s skin. In fact, it’s recommended to not use vinegar, lemon, or bleaches on animals, as these products are too acid or too alkaline and will damage the coat.