The stems, edges and base of the leaves have tiny hairs which contain a poisonous sap, and the slightest touch will cause very painful blisters and severe skin irritation. Un-shaded habitats with high soil nitrate levels (for example, riverbanks, roadsides and waste ground) tend to produce greater quantities of toxins in the plant.
Contact with the cut material in sunlight produces a skin reaction in almost all cases. Blistering symptoms occur 24-48 hours after exposure, and dense colouring is visible after three to five days. This may persist for six years or more.
Cut material remains active for several hours after cutting. Protective clothing must be worn when treating this plant because the hairs can penetrate light fabrics