I had it bad for a while, and Selson Blue, Head and Shoulders, and T-Gel didn't do a thing to it. I ended up getting a shampoo called Nizoral. It's not cheap, but for the money I spent on other cheaper stuff, I could have probably bought two bottles of the stuff. It has a different active ingredient than the other shampoos but it worked incredibly well.
Yeah, the Nizoral is really ketoconazole, which is an antifungal. It kills the fungus that causes your seborrhea (which is what causes your "dandruff").