There are two forms of melatonin used in supplements. The first one comes from the pineal glands of animals. The second type is synthetic. It's generally accepted that this is the most acceptable form. Although the synthetic version is the most popular some supplements contain animal-derived melatonin. The label on the bottle should tell you which version is being used. If it doesn't, unless you don't mind using the animal-derivative, it may be best to choose an alternative instead. Obviously, if you are a vegan or vegetarian, the synthetic version will be the only one suitable for you. Apart from the issues with animal rights, natural melatonin presents another problem. It may contain undesirable viruses. One way or another, if you are thinking of using melatonin, the synthetic version is best.