Vitamin C

Vitamin C does two jobs in a supplement like this: it is an antioxidant in its own right, and it is required for your body to actually make collagen — which is why it is nearly always included alongside collagen peptides rather than sold separately. It is water-soluble, so what you do not use is excreted and very high doses mostly produce expensive urine. A sensible daily amount alongside a normal diet is the useful range.