Depending on the cause, there may be other symptoms such as loin pain, burning on passage of urine, or passage of a kidney stone. The urine may look pink or red or there may be clots. Sometimes this may be accompanied by pain. The best way is for your doctor to test the urine for blood with a dip-stick or under the microscope. Other things often lead to the urine looking dark and more rarely, red.
Some medicines do turn the urine red.
Red colouration in the urine may also come from pigments such as that in beetroot. Although many things make urine look dark in colour, it is always worthwhile seeing your doctor if you think you've seen blood. All of these can be treated and in all cases, the shorter the time between discovering blood in the or urine and determining the cause, the greater the likelihood that treatment will be effective. The doctor will first confirm that blood is present using a dip stick and probably sending a sample to the laboratory. This will also determine if infection is present.