Constipation doesn't occur often in guinea pigs. When it does, though, it can be serious. Guinea pigs need to eat certain droppings to properly digest their food. If they can't eat them over a long period of time, they can get very sick. Constipation is indicated by stools that are small, round, hard, dark and often stuck together.
Check your guinea pig's stools every day. They should look like long brown pellets. If your guinea pig is not producing any pellets, make sure he is getting enough water and fresh fruit and vegetables.
Monitor your guinea pig. If she is sitting in a hunched position and seems stiff, these are signs that she could be constipated. Offer your guinea pig a fresh apple peel.
Call the vet and make an appointment if your guinea pig hasn't passed any stools a couple of hours after eating the apple peel and still seems uncomfortable.
Place a couple of drops of olive oil on a leaf of romaine lettuce and offer it to your guinea pig.
Fill a clean syringe with 1 tbsp of mineral oil or olive oil if your guinea pig refuses to eat the romaine lettuce.
Wrap the guinea pig gently in an old towel so her head sticks out and nothing else. Place her on your lap and stick the syringe behind her front teeth. Don't force it. Depress the plunger slowly. Expect some to go on the towel, your lap and the front of the guinea pig. As long as the guinea pig swallows a bit of it, she should be fine.
Keep a close watch on your guinea pig and talk encouragingly to him. If the guinea pig wants to sleep, let him sleep.
Take your guinea pig to the vet if she still doesn't pass any stools.
Guinea pig constipation is technically called "motility issues." The kind of constipation described is not to be confused with anal impaction, which happens mostly to male guinea pigs.
Do not use Iceberg lettuce. That will give your guinea pig gas and only make matters worse. Do not ignore constipation in guinea pigs; it usually does not go away on its own.