Guinea pigs cannot make their own vitamin C, and the water drops do not work

Guinea pigs are in a very small club with humans and primates. They're missing the enzyme that makes vitamin C, so every bit of it has to arrive in what they eat. Get that wrong and you get scurvy, and it's not some rare textbook thing, it's one of the most common problems vets see in pet guinea pigs. The number the veterinary references give is roughly 10 mg per kg per day for a healthy adult and around 30 mg per kg for young, pregnant or sick pigs. In practice that usually gets quoted as about 20 to 25 mg a day for a normal adult. Now the part that catches people, because it's sold in every pet store. Those vitamin C drops you add to the water bottle are close to useless. Ascorbic acid falls apart fast in water, light and heat, and the figure that gets cited is that after about 8 hours you're down to something like 20 percent of what you put in. On top of that it changes the taste, so some pigs drink less, which is its own separate problem. What actually works is fresh food and fresh pellets. Bell pepper is the easy daily win, red or green, and by weight it carries more vitamin C than orange does. Parsley and cilantro are good too. Guinea pig pellets are fortified but the vitamin C degrades on the shelf over a few months, so buy small bags, look at the mill date rather than just the expiry, and resist buying in bulk to save money. If you want to supplement directly, use a chewable tablet at a dose a vet gives you, not the water additive. Early scurvy looks like a pig who doesn't want to move, a rough coat, swollen or sore joints, a limp, poor appetite, and small wounds that won't heal. It's very easy to file all of that under getting old.