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Constipation in guinea pigs is usually caused by a lack of moisture in the diet. If the vitamin C you are giving is the kind you put in the water, I would dump it and give him plain water. It's better to give the vitamin C by syringe (into the side of the mouth, not straight down the throat) or use children's chewable tablets - about 25 mgs. Make sure he has clean, plain water available. Then, you can give him some apple with the peel. Rinse the apple before giving it to her, and leave the rinse water clinging to it. You can also put a drop or two of olive oil on the greens. A great variety of fruits and vegetables should be included in the diet. Spinach is not a good choice. It has too much calcium oxalate, which can lead to bladder stones over time. This site has great information on diet, including the best produce to feed:
If the above measures don't help, your guinea pig may be suffering from impaction. However, that usually results in abnormally shaped stools, rather than none at all.
This problem is located in the perineal sac, and is common in middle-aged and older guinea pigs. Even if that isn't the problem right now, you'll want to be familiar with it in case it occurs in the future.
If that's the problem, you can either take your guinea pig to a vet for treatment, or if you're not squeamish about it, you can do it yourself. If you decide to go to a vet, here is a link to a directory of vets who are familiar with guinea pigs:
If you decide to take care of it yourself, you can best learn how through pictures. This site takes you through the process step-by-step with photos:
If the increased moisture and olive oil don't solve the problem, and you don't think it's impaction, you will need to see a vet. There could be a blockage of some sort. Also, if your guinea pig should stop eating, that is a medical emergency, calling for an immediate vet visit.
If you have more questions, just let me know. I hope your guinea pig will soon be passing normal droppings.
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