Guinea pigs lack an enzyme that would otherwise catalyze the conversion of glucose into vitamin C. Thus supplementation of the diet is necessary. Diets deficient in vitamin C can result in scurvy, immune system disorders, defective collagen formation and wound healing, orthopedic problems and bleeding disorders.

I recommend that you give your guinea pig vitamin C supplements that can be added to the water daily and give foods that have a relatively high vitamin C content. Since vitamin C is water soluble, it is not a concern that you are giving too much. It is a serious problem for guinea pigs when they get too little.

Much of the vitamin C that is in a guinea pig's pelleted food leeches from the food before it is fed to the pet. Vitamin C degrades (oxidizes) when exposed to air, so fresh supplement and water must be given each day. Chlorinated water should not be given because it reacts to degrade vitamin C
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Feeding a balanced diet with foods high in vitamin C is a good husbandry practice. However, I recommend that vitamin C should be added to the water regardless of the potential vitamin C content of food.

The required daily dose of vitamin C is 10 to 30 mg/kg; this requirement increases when guinea pigs are pregnant, nursing or sick. Talk to your veterinarian about how much vitamin C you should give your guinea pig and how to mix it proportionally in your pet's water bottle.