Diarrhea is most commonly caused by a sudden change in diet, feeding too much fresh food, and feeding too much moist food. It is normally easy to treat, however it can also be a symptom of the far more serious condition wet tail. If you suspect that your hamster has wet tail you should take it to a veterinarian as soon as possible.
Diarrhea can be identified by droppings that are soft, watery, and a lighter color than normal, while there may also be wet and matted fur around the tail. Your hamster should appear healthy in all other respects though, whereas wet tail has more wide-ranging symptoms.
Diarrhea can usually be resolved by removing fresh food from your hamster’s diet, and only feeding hamster mix, until the diarrhea has cleared. At this point you can start to slowly reintroduce fresh food. Exceptions are arrowroot and raspberry bush leaves which can help recovery from diarrhea. You should keep a hamster warm if it has diarrhea, and make sure it has plenty of water to avoid dehydration.
If your hamster does not recover after 2 days, if the diarrhea gets worse, or if other symptoms occur then you should take your hamster to a vet immediately.
You can help prevent occurrence of diarrhea by only feeding your hamster fresh food as a treat once or twice a week, and by introducing any new foods gradually. A good quality hamster mix should provide your hamster with the nutritional health it needs.