The Healthy reports that grapefruits and some other citrus fruits can be harmful to some people but are relatively healthy for most who decide to snack on them. Some citrus fruits can also interfere with medications, so if you are taking a prescription, check with your doctor to see if the two can mix. Live Science came to a similar conclusion, writing that oranges should be enjoyed in moderation, telling their readers that there could be some unwanted side effects if they eat too many. 

Oranges naturally have a large amount of fiber, and if eaten in large quantities, it can affect a person's digestive system. Translation: Make sure there is a toilet near by, as some of the more dreadful side effects can be cramping and diarrhea. Citrus fruits are generally go-tos for dieters since they are low in calories, but Live Science encourages its readers not to eat several a day. The Mayo Clinic even revealed that it is possible to have too much vitamin C throughout your day. Citrus fruits are also high in acidity, which could lead to heartburn. But if you have a clean bill of health, an orange or tangerine a day won't kill you.