I'm pretty sure the collagen supplements are a bit of a scam
Collagen is a protein so surely when you digest it, it's broken down into its amino acids components like any other protein and they're taken up, or excreted if they're in excess.
I can only think they 'work' because if your body is making a protein and a specific amino acid is limited, that protein cannot be made by the body. Perhaps the collagen supplements provide amino acids which your body lacks through a normal diet...
I'm just trying to say I think they use the word 'collagen' because it's associated with good skin so they charge more, but it's not directly increasing your collagen production etc.
.. if it's helping then fair enough

I take primrose oil with b6 and vitamin d and omega 3 when i remember