Hi and welcome
Can you please detail what your piggy has been diagnosed with, what medication he is on, what diet and how much vitamin C you are feeding extra to the diet? How long have you had your guinea pigs for?
Has your vet asked whether your piggies were already on vitamin C supplements before recommending more?
Please be aware that while guinea pigs can't make their own vitamin C, they are getting enough in of it in a healthy, mainly hay based diet with a small amount of judiciously mixed veg that provide a wide range of nutrients, vitamins (including vitamin C) and trace elements. What most people, including general vets, are usually not aware of is that grass and hay are actually high in vitamin C, which is the reason why guinea pigs have never had the need to make their own in the first place as they have always got enough from their main food source dry grass (hay) and fresh grass, which should make over 80% of the daily food intake while veg is ideally only about 10%.
If you consistently overfeed on high levels of vitamin C, the body of your piggies is adapting to that and is ironically reactingwith symptoms of scurvy (vitamin C deficiency) whenever this high level drops during an illness or for some other reason. It would be good if you reviewed your diet once your piggies are better again to see whether you could improve it.
While a 2-3 weeks vitamin C booster on a normal diet is usually all that is needed for an ailing guinea pig, it is very tempting when you are worrying about your piggies to throw the kitchen sink at them; but that can actually do more harm than good.
Please concentrate for the long term on a good, healthy hay based diet for lasting good health and a longer life span rather than stuffing your piggy full of artificial products and unbalancing the diet further to their long term detriment. You will have to manage the transition gradually and slowly in order to avoid scurvy, however.