Thank you for all the suggestions. I'm going to take my time and look around since I've never bought any shampoo for a guinea pig. I'll make sure that I don't get anything with any chemicals, pesticides, perfumes or any other additives. I appreciate all the good advice that everyone has taken time to give me, because I want my guinea pig to have the best possible care that I can provide for him.
I'm sorry for the confusion that I caused. My current guinea pig is not sick, and I'm not shopping for any medication for him. I saw the Marshall brand medicine for URIs when I was looking for shampoo on the Petco website, and I was just curious about it.