Semicarbazide (SEM) is a product of the metabolism or degradation of the antimicrobial veterinary drug nitrofurazone. The drug is prohibited for use in food-producing animals in the UK, and detection of SEM is used as an indication of nitrofurazone abuse. SEM was reported in several heather honey samples tested in Scotland and Northern England in 2009, and also in two samples of wild forest honey from New Zealand in 2011. The hives from which these samples were taken were strongly considered to have been free from nitrofurazone treatment, and therefore a natural source of SEM was suspected. The natural formation of SEM has been demonstrated in certain shellfish, seaweed, eggs, and whey, with arginine and creatinine proposed as its precursors. This paper reviews the natural formation of SEM in foods to help to identify a natural source in honey. A possible source is a sudden increase in arginine levels in heather pollen shortly before and during the production of the affected honey. Other possible sources of the SEM are unidentified precursors and environmental contaminants, including urine from sheep or wild animals.