Just like us, guinea pigs cannot produce Vitamin C by themselves and may require supplements for healthy growth and well-being, if their diets do not contain sufficient amount of the nutrient.
The lack of Vitamin C will lead to scurvy. Look out for signs of scurvy, such as weakness, lethargy, loss of appetite, swollen or painful gums, and general skin infections. In addition, if the animal whines when being handled, or if its coat feels rough, these may be signs of Vitamin C deficiency.
Restoring the regular intake of Vitamin C will help treat and prevent scurvy. Administration of Vitamin C can be via pellets, tablets or liquid mixtures. However, since Vitamin C breaks down easily at high temperatures and when exposed to light, take note to store them in cool, dark places. You may also crush some Vitamin C tablets and sprinkle over moistened fruits and vegetables before feeding them to your guinea pigs. Some owners prefer to add Vitamin C drops to their pets’ drinking water but it is not recommended. Vitamin C breaks down quickly when exposed to air or water, and it changes the taste of water. This may result in the guinea pigs consuming less amount of water. However, take note to replace drinking water daily.
Fruits and vegetables are the best source of natural Vitamin C for guinea pigs. Examples include guava, orange with peel, parsley, broccoli, kale and mustard greens. An adult guinea pig will need about 25mg of Vitamin C daily.
A pregnant guinea pig will need more, about 30 to 40mg. (Source: http://www.peteducation.com/article.cfm?c=18+1800&aid=620) The correct dosage will depend on the guinea pig’s size, diet and health condition. Avoid giving them multivitamins as this may lead to feeding too much of some vitamins and minerals. Always seek a vet’s opinion when in doubt.