Guinea pigs are great pets to have, but they are very unique in that they require vitamin C to be added to their diet. They are missing an enzyme that we have that is necessary for the body to make the vitamin. Most guinea pig diets have Vitamin C added to them, Vitamin C will only remain active for 90 days past production and if stored correctly. Make sure to check that the date on your bag isn't more than 90 days old. Some foods that can be given to supplement Vitamin C are dark, leafy greens, cabbage, red/green peppers, asparagus, broccoli, peas, tomatoes, kiwi, and oranges. You can also give dandelion greens as long as no pesticides, fertilizer, or herbicides have been applied. If your pig is showing signs of anorexia, depression, weight loss, swollen joints, nasal discharge, or ruffled fur, get to your local exotic vet quick.