Crab apples are not poisonous to humans if eaten properly. The flesh of crab apples is safe and edible, but the seeds contain cyanogenic glycosides, which can release cyanide when metabolized in the body. Therefore, it is important to avoid eating the seeds and core of crab apples. Eating a few seeds accidentally is unlikely to cause harm, but consuming large quantities of seeds could potentially lead to cyanide poisoning. Properly avoiding the seeds allows crab apples to be safely eaten, though they tend to be more sour or bitter than regular apples.
