I tried Viviscal a few years ago. They work.
My hair grows very fast without any help, but I wanted it to grow even faster after asking a hair stylist to cut an inch and she cut over six inches instead. I tried Viviscal then.They really work. I had to stop taking them because I dye my hair from black to very light ash blonde and I wait a month for touch ups during which my hair grows about an inch, but after taking Viviscal my hair grew almost two inches in that first month and the color didn't look even. I took them for another month, had to touch up twice, but by the third month my hair was almost back to the lenght it had before.
One of my friends took them after I did. Her hair grows very slowly and while her hair grew faster with Viviscal it only grew half an inch per month. But for her that was a big difference.