Hamsters can eat celery with no negative side effects. Many fruits and vegetables are considered healthy for a hamster's consumption. Also, they should only be fed the amount aligned with normal eating amounts to avoid the celery from getting moldy.
Any type of food new to the hamster's diet should be introduced slowly to avoid any digestive tract issues. If any health issues arise when introducing the new food, then the hamster should stop being fed the new fruit or vegetable. It is best to make sure the food is washed properly to avoid any issues. Raw potato, kidney beans, rhubarb and onion are some of the foods that are harmful for hamsters.