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human
I do not believe that body wash is much different than shampoo. CMV

I don't think that substituting body wash for shampoo (or vice versa) will have negative effects on hygiene or hair quality. Other than scent compounds, I think they're basically the same thing--they remove dissolvable organic compounds from the skin/hair. To that note, I think bar soap is similar as well, though I'm less sure on that point. 

Sources: first, anecdotally, I've used bar soap, body wash, and shampoo on my hair. All (with a good conditioning after) left my hair feeling silky and smooth. 

Second, if you're removing organic compounds, then you're removing organic compounds. Is there really that much of a difference to the individual chemicals used? I can also see selenium sulfide shampoo as being something different altogether--I've used it as a rub for a fungal infection.

Note that I'm not including conditioner or shampoo plus conditioner. Those clearly have a point--they reapply the slick oil coating to hair after the shampoo washes it off.
gpt
For the most part you're right. Body wash and shampoo are a lot more similar to each other than either than they are different. Especially if you compared them to, say, bathroom cleanser, or dishwasher detergent. And it's certianly possible to find a body wash that you can use as both. But that doesn't mean that *every* bodywash is the same as *every* shampoo. Or if one body wash works as shampoo for you it will work for everyone else.

If you used dishwasher detergent in the shower the results would rapidly become unpleasant. This is because dishwasher detergent is actually *too good* at dissolving oil and will deplete your natural oils beyond what most lotions and conditioners would be able to make up for. This is called "harshness".

Harshness depends on two factors: concentration and chemical identity. Certain soaps are just harsher than others, and no concentration will change that.

Bodywashes tend to be a little bit harsher, so that a "mild" bodywash might be as harsh as an "average" shampoo. The result is that if you have problems with dry hair the average bodywash is probably going to be a little too harsh, even with conditioning. It would probably take repeat uses before you notice, though.