Atropine molecule, molecular model. Atropine is a chemical produced by the deadly nightshade plant (Atropa belladonna), and is one of the chemicals responsible for the plant's toxicity. The chemical blocks nerve signals in the parasympathetic nervous system, which controls unconscious body actions such as heartbeat, sweating and breathing. It causes a rapid heartbeat, dry mouth and skin and irregular breathing, and can be fatal. Atropine is widely used in medicine. It is used to treat a slow heart rate or heart block, to dilate the pupil of the eye, and as an antidote to organophosphate poisoning (such as that from nerve gases). Atoms are colour-coded: carbon (grey), oxygen (red), nitrogen (blue) and hydrogen (white).