I'm one of the small minority, along with Zany, who can successfully use soap as shampoo on my hair. I never did a cold water rinse as Zany described, but did use vinegar rinses. Really, I believe the issue has more to do with hair type, frequency of washing, perhaps climate and diet, etc. FWIW, my husband was able to use soap on his hair for a few years before he experienced super dry and flaky scalp, and some hair fall - and his hair was already quite short, too.
Given all the issues others had, I decided it wasn't worth risking eventual problems, so I switched back to liquid shampoo for a bit, and then on to true shampoo bars. They truly are the same ingredients as what you will find in most liquid shampoos, just without the water. If you can't get the ingredients there, that is a bummer.