Guinea pigs cannot make their own vitamin C and must get it from their diet every day — via fresh grass-based pellets and vitamin-C-rich vegetables — or they develop painful deficiency (scurvy).
Vitamin C degrades in air and water, so a fresh daily portion of pellets and greens matters — don’t just top up a bowl, and don’t rely on vitamin drops in the water bottle.
Guinea pigs are unusual among small pets in this; most others make their own vitamin C.