There are several types of elderberry plants, and not all the berries are safe to eat, especially when raw. (Note that elderberry is different from elderflower.)
According to the United States Department of Agriculture [link to pdf], "Only the blue or purple berries of elderberry are edible. Edible berries and flower are used for medicine, dyes for basketry, arrow shafts, flute, whistles, clapper sticks, and folk medicine. The active alkaloids in elderberry plants are hydrocyanic acid and sambucine. Both alkaloids will cause nausea so care should be observed with this plant. Elderberries are high in Vitamin C. The red berries of other species are toxic and should not be gathered."
In one instance [link], several people were sickened after consuming raw elderberry juice.
"Raw Elderberry a Danger to Health (especially for kids)" [link]