Guinea pigs will naturally drink more when it's warmer, or when they have less veggies available. But if a guinea pig suddley changes its habits and starts drinking a lot more than usual, then you need to get it checked out as it's often a sign that something isn't right. For example diabetes or problems with the kidneys can cause excessive drinking, and a lot of guinea pigs with any kind of infection drink more.
If you can, collect a urine sample to take to the vet with you. Put the guinea pig in a clean plastic tub until he pees, then pour it in to a clean container. A simple dipstick test can rule out a few problems.
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