What it is
E123 is amaranth, a synthetic dark-red azo dye. Named after the amaranth plant (because of the colour resemblance) but not derived from it.
Why it matters
Used in caviar (the imitation kind), aperitifs, soft drinks, decorations on cakes. Banned in the US since 1976 over carcinogenicity concerns; still permitted in the EU/UK at limited levels.
What to do
Always vegan.