Yeah, I read online a lot about the fungal aspect of it all. I am just fed up with trying to find shampoos that I am not allergic to AND help with dandruff. It's hopeless. I might as well just make something of my own!
I find it interesting that over roughly the same period of time, dandruff and peptic ulcers went thru a stage of being believed to have an infectious/microbial cause, then being believed not to have such a cause, and then again believed to have such a cause. In neither case is the microbe sufficient to produce the condition, because most people are colonized by it, but most people don't have the condition. However, if you get rid of the microbe, you do get rid of or at least improve the condition.
Nobody to my knowledge is using undecylenate as a dandruff treatment, the industry having moved on to stronger antifungal agents. Nobody's going to invest the time to get FDA (or other governments') licensure of this labeled use of an antifungal when athlete's foot etc. treatments have moved on to stronger ones, but I suspect undecylenate will work at least fairly well when formulated well. Undecylenic acid moieties are being used in some surfactants to help preserve toiletry formulas against mold, as long as it's a secondary usage (the surfactant having another primary purpose in the formula) so they don't need to get EPA registration as a fungicide. Similarly someone could use undecylenate in a soap-based shampoo bar, just not claiming it as an active ingredient; they could even market it as a dandruff shampoo with another, licensed, active ingredient.
But although antifungals are good for
preventing dandruff, there are still other ingredients better at
removing dandruff than general detergents. Some ingredients like salicylic acid are keratolytic, kind of blasting the outer layer of skin to remove dead stuff
now instead of waiting for it to form dandruff. Others are keratoplastic, softening and smoothing down the skin so it doesn't flake off so fast. In experiments on pigs at Procter & Gamble ~25 yrs. ago, Bisset & Mao showed palmitamidropropyl (cetamidopropyl in their terminology) betaine was a good secondary surfactant for removing flakes & smoothing skin, but they never to my knowledge used it in shampoo, only in Ivory Dishwashing Liquid, and then only for a couple years; I think they decided it was too expensive.
