EDDP is the most important metabolite of methadone. It is excreted in the bile and urine together with the other metabolite EMPD. EDDP is formed by N-demthylation and cyclisation of methadone in the liver. The part of the unchanged excreted methadone is variable and depends on the urine’s pH value, dose, and the patient’s metabolism. Therefore, detection of the metabolite EDDP instead of methadone itself is useful because interferences of the patient’s metabolism are avoided. EDDP can be detected within 4 to 6 hours after use. It can be cleared by the body within 2 to 3 days after use.