Vinegar Rinse and Finer Hair
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My brother had this thinning happen to him before when he started using Nioxin dandruff shampoo (I think that was the brand) and he used a thickening shampoo to fix it. I started doing vinegar rinses, and I'm wondering if maybe that can be the cause (although it does help my scalp)? I would really, really appreciate advice!
The fact that you have said that the hair hasn't been falling out or breaking leads me to believe that your hair isn't getting weaker, just more compact. Weakening of the hair would lead to more breakage and noticeable signs of damage. The tightening of the cuticle actually makes the hair more resistant to potential damage.
However, if you aren't happy with the texture of your hair, you should consider reducing the frequency with which you use the vinegar rinses. Since the reason for using the vinegar is to deal with dandruff flakes, perhaps you could add in a dandruff treatment shampoo to your hair care routine. Simply replace every third shampoo session or so with the dandruff shampoo and cut back on your vinegar rinses by half.
You may need to play around with the schedule of uses, but you should be able to find a balance between your scalp management needs and keeping your hair feeling thicker and fuller.
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See also:
Dandruff causes and treatments
How to see the difference between broken hair and new growth
How to make your hair look fuller and thicker