Watermelon is full of beneficial nutrients but, in large quantities, it also causes some uncomfortable side effects. Since watermelon is rich in fiber, it may trigger flare-ups in intestinal disorders such as Crohn's disease or irritable bowel syndrome and cause gas, bloating and diarrhea.
Watermelon also contains over 6,888 micrograms (6.8 milligrams) of lycopene, a powerful antioxidant known to reduce stress, reduce cholesterol and in aid in preventing cancer. However, according to WebMD, research shows that lycopene might worsen existing prostate cancer. Other studies, noted by Fitday, also indicated that consuming large quantities of lycopene, over 30 milligrams per day, might cause allergic reactions and immune system imbalances.