Can you use dog shampoo on cats?
The short answer is 'not always'. If you check the label, most (but not all) shampoo suppliers will say if the shampoo is also safe for use on cats and kittens. Some may be for dogs and cats only, others will specifically mention 'puppies and kittens' (if your cat is under 12 months old most class them as kittens still rather than adult cats).
There are a few reasons you shouldn't go ahead and use a dog shampoo on a cat, unless you know it's safe for cats. One is an ingredient called permethrin. It's common in dog shampoos that help treat pests like fleas, but for cats that ingredient is poisonous (note some flea shampoo like Fidos is safe for dogs and cats).
Another reason is how different a cat's skin is to a dogs. A cat has almost paper thin, loose skin, and the pH of their skin is more acidic than dogs. Dogs average a pH of 7.5 - cats pH averages 6 - humans average 5.5. The difference between each move up and down the pH scale is huge in real terms (up to 150 times more or less acidic).
Small changes to a cat's pH can make a big difference, plus cats groom at an entirely different level to dogs. An incorrect skin pH can lead to cats getting itchy and irritated, dry flaky skin, or overly oily greasy skin. The wrong pH can also make cats more likely to get infected with fleas and other parasites. Washing cats should be a last resort to help them maintain the correct skin pH naturally.
Before you use 'dog' shampoo on your cat, check the label or ask the supplier. It should mention 'safe for dogs and cats' or 'puppies and kittens' not just dogs or puppies. Even if it says 'pets' it's safer not to assume that includes cats. It should specifically mentions cats or kittens.
Keep in mind also to check the label of the exact formula of shampoo you want to use - not just the brand as a whole. One brand might have formulas that are safe for cats, but might have a flea shampoo that isn't. Same brand, different safety.