If your guinea-pig is losing enough weight that you would actually notice, it usually means something is wrong. Generally owners do not recognize slight fluctuations in weight when they see their guinea-pigs every day. While guinea-pigs do fluctuate in weight, there is probably something wrong when your pig loses more than an ounce or two in any given week, and you need to have your pet thoroughly examined by a vet. If your guinea-pig’s loss of appetite is accompanied by the signs of a cold or he has trouble breathing, this may indicate an Upper Respiratory Infection (URI) or even Pneumonia, and this will require medication.

One common reason for a guinea pig not eating is Malocclusion of the teeth. This occurs when the teeth are not aligned, so they do not wear each other down and this leads to them becoming too long for the guinea-pig to eat properly. Often affected animals have the appearance of having a slightly open mouth at all times. Note that this can occur not only to incisors but also to the back molars. Even if you have your vet trim the incisors make sure he also checks the molars.

Constipation in guinea-pigs is a serious issue which will not usually resolve itself without action, and can be the result of a poor diet or an underlying illness. Your guinea-pig’s diet should contain plenty of water, fresh greens and hay. If your pet’s diet did not contain fresh greens you should introduce them slowly to avoid loose stools. If your pet’s diet is appropriate you can try adding a little olive oil to his favorite treat and feeding him directly. If this does not resolve the issue within 24 hours you must have him thoroughly examined by a vet.