Urine is usually not red after eating watermelon. Urine that is red may be caused by taking food or medication, or it may be caused by certain diseases. Watermelon is a common fruit, rich in water, eating watermelon may cause an increase in urine, generally does not cause the urine is red. 1. caused by food or drugs: for example, taking too much red dragon fruit, or taking drugs such as rifampicin, will cause the urine color red. 2. Certain diseases: Common diseases include kidney stones, ureteral stones, bladder tumors, prostate tumors, leukemia, hemophilia and other diseases. Urine is red, food or drug factors, generally no treatment; disease caused by the need to go to the hospital in a timely manner to check, clarify the cause of the disease, active treatment.