Featured photo: Cinnamomum zeylanicum plant (true cinnamon, Sri Lanka cinnamon) Photo: Sudhir Ahluwalia
Cassia and cinnamon both contain coumarin. Coumarin is toxic to humans when ingested regularly. Prolonged use can cause liver inflammation and jaundice. The maximum recommended daily intake of coumarin is 0.1 mg per kilogram of body weight. 1 teaspoon of cassia powder contains between 6–12 mg coumarin. European Food Safety Authority limits the quantity of cassia that can be safely ingested.
Three essential oils are extracted from cinnamon trees: eugenol from the leaves, cinnamaldehyde from the bark, and camphor from the root. Leaf oil yield is 0.7–1.2 percent and the eugenol is used to synthesize vanillin and converted into iso-eugenol for use as a flavoring in confections.
The medicinal properties of cinnamon have been scientifically studied, and preliminary results suggest it has antidiabetic properties (Ranasinghe et al., 2012). Other studies indicate that cinnamon can reduce cholesterol, thus making the species cardio-protective (Shan et al., 2007).
It is used as a tonic and sedative in childbirth. When applied to the skin, cinnamon oil causes blood to rush to the applied area, which helps nourish the skin and produces a tingling sensation. It also produces a temporary plump look that minimizes wrinkles and lines, especially around the eyes. Applying oil with a few drops of cinnamon essential oil helps relieve itchy scalp and acne.
More on sources of cinnamon and cassia, botany, medicinal and other uses, history, etc here. There is a section devoted to these species.