Can melatonin cause false positive for alcohol in urine test?

Melatonin is a natural substance your body creates to regulate our sleep cycles. It is more likely that a bacteria contaminated your sample and is responsible for the false positive. The method of collection may be the reason. Since you did not say if it was a clean catch urine sample, meaning that you were given a couple cleansing cloth packets and were instructed how to clean yourself before giving the sample then releasing a small amount of urine, stop then proceed to catch your urine, it is difficult to answer more specifically. You can talk to your doctor about the result and about your factors to best understand how this may have happened.