This my third time using a GNC Collagen supplement. I had been experiencing profuse hair shedding. Just as with the previous occasions, within 2 weeks of starting, there was a noticeable reduction in the amount of hair that was being shed. Over the last 4 months the hair shedding rate has decreased significantly. I can't say there is an increase in thickness after only 4 months (although my derm saw increased strand thickness upon analysing my hair) but it stands to reason that eventually, I'll have more growing hairs than shedding hairs and the thickness and density will improve.
Why did I stop before? One reason was expense, another was thinking that the protein in my diet was sufficient and yet a third reason was just to prove to myself that I can prevent my hair from shedding by using other methods.
The Collagen formula worked for me. It likely won't work for you if your issue isn't related to collagen metabolism etc. This time, I'm not giving them up.