Vitamin C deficiency, also known as Scurvy, is extremely common in guinea pigs. Guinea pigs require at least 90 mg of vitamin C every day.  An unhealthy guinea pig may need 150 mg or more of vitamin C daily so increase the dose if you suspect your guinea pig is becoming sick.

Guinea pigs cannot synthesize their own vitamin C. Symptoms of vitamin C deifiency in guinea pigs include rough fur, loss of appetite, difficulty eating, diarrhea, problems with wounds healing, lameness, joint sweling, bruising, gastrointesitnal stasis, bumblefoot, and increased susceptibility to bacterial infections (i.e. respiratory infections). Prolonged deficency can affect skeletal and tooth development leading to brittle bones and dental problems. 
Many people try to satisfy a guinea pig's need for vitamin C feeding fresh fruits and vegetables. However, when you look at the vitamin C content of many fruits and vegetables you can see that it takes a lot more than you'd expect to reach their vitamin C needs. Most are going to require much more fed to your guinea pig than their dietary recommendations. 
This chart shows the vitamin C content of common leafy greens fed to guinea pigs. We recommend guinea pigs receive 1/2 to 1 cup total of leafy greens daily. As you see here, in order to receive enough vitamin C from greens a guinea will have to eat more than is recommended. This can lead to diarrhea and other digestive problems. 
This chart shows the vitamin C contect of many vegetables fed to guinea pigs. We recommend only a small amount of these vegetables (less than 1/4 of a cup) be fed only 2-3 times a week. Again, to get the amount of vitamin C needed you will have to feed more than is safe for your guinea pig to eat. 
This chart shows the vitamin C content of many fruits fed to guinea pigs. We recommend fruit be fed to guinea pigs only as a rare treat as they contain lots of sugar which can lead to serious problems if overfed. As you can see, it isn't possible to safely feed enough fruit to provide enough vitamin C to your guinea pig. 
So what can you use? We recommend using a product designed for guinea pigs, Oxbow Natural Science Vitamin C. This is a small "cookie" like treat that supplies your guinea pig with the extra vitamin C they need to be healthy. We carry this in our store area.
You may also use a liquid vitamin C for humans.  There are many different brands of liquid vitamin C sold in health food stores.  Most have about 250 mg per teaspoon so a dose of 1/2 teaspoon a day is right for a guinea pig. If you are using a human product and are not sure that it is the same dose as this, please call and let a doctor know what you are using and so you can be sure you are giving the right amount.
Water based vitamin C drops can also be found at pet stores. However, we do NOT recommend these for a number of reasons. First, because these dilute the vitamin C into a very low concentration in the water your guinea pig would have to drink rediculously large amounts of water to reach their needs every day. Second, they make the water taste funny and will actually discourage your guinea pig from drinking enough water. And third, vitamin C is sensitive to exposure to light. Since most water bottles are clear to white opaque light can penetrate to the vitamin C and inactivate it - rendering it useless. 
If you have more questions about vitamin C supplementation in guinea pigs, please feel free to contact us.