@HealthMed Guides to plants have a large number of entries labelled as 'medicinal use' and 'poison'.
The ancient Greeks recognized this overlap, in using the same word for drug, poison and magic charm: pharmakon. This remains in current use in the word pharmacology - the study of drugs of all types.
The Temperate House at Kew Gardens, the largest surviving Victorian glasshouse in the world, has a tantalising display of medicinal plants, for most the scientific name the only clue to properties. Nearby there are two examples of contrasting properties of plants.
Artemisia [in this case A. austriaca - Austrian wormwood] is there as scrawny, pale green, ground cover. This plant is thought to be named after 4th century BC queen of Persia and botanical expert Artemisia, or after Artemis, sister of Apollo and Greek goddess of the wilderness, the hunt, fertility and both bringer and treater of disease in women. This bitter herb is said to have been used by the ancient Greeks to treat parastic infestation by intestinal worms. A large number of medicinal properties are claimed for wormwood species. Artemisinin, the extract of Chinese Wormwood, is used as an anti-malarial however resistant strains of malaria are now being reported. Recent experimental evidence suggests that artemisinin may have anti-cancer effects through limiting proliferation and cause programmed cell death (apoptosis) in certain sub-types of cancer cell. Other research indicates that artemisinin may induce cancer cell resistance to other types of cancer treatment. Much more work is needed on effectiveness and safety of artemisia extracts in the clinic.
Angel's Trumpet [Brugmansia aurea] in contrast has more sinister properties. This tree, native to sub-tropical South America from Colombia to Ecuador, has delicate, orange, trumpet-shaped, hanging, lemon-scented flowers. All parts of the plant are considered poisonous through antagonism of muscarinic cholinergic receptors and other effects of tropane alkaloids, including the racemic mixture atropine, its laevo-stereoisomer hyoscyamine [M1-antagonist], and scopolamine. Actions include belladonna-like effects, hallucinations, sedation and pupil enlargement, which may be symmetrical or asymmetrical. Local shamans are reported still to use plant extracts to aid their ceremonies through inducing visual hallucinations. Historically it is believed that in the Americas extracts of this plant, combined with alcohol and tobacco, were used to induce sleep in slaves or wives before their immolation after the death of their king.