Some of the human shampoo formula can be a little "harsh", which can dry up the skinn, causing skin irritation, flakyness or can cause upset stomach when swallowed :) Johnson & Johnson's baby formula is one of the "gentle" ones.
Human shampoos (with the possible exception of baby shampoo) are all much harsher than dog shampoos.
I use Buddy Wash Shampoo and Buddy Rinse Conditioner. Both are soap-free, natural with a coconut base and a fresh lavender and mint scent. Though it is a lot more expensive than my own shampoo, Charlie only gets a bath every few months when he gets a little stinky, so a bottle will last well over a year.
It's better to stick to shampoos for puppies/dogs than human shampoo. As our members have stated, it's just not suitable. I used to buy shampoo from the petshop but now I look for puppy/dog shampoo without chemicals.