Both German and Roman chamomile are used for medicinal purposes. Other species of chamomile contain less substances considered useful for humans, but are still edible. I have, however, found information that there is one species of chamomile which is slightly poisonous to animals, and definitely should not be fed to guinea pigs. This is so called stinking chamomile, Anthemis cotula. Fortunately, it has an unpleasant smell, which makes it easy to recognize and avoid.