Strychnine. Computer graphic of a molecule of strychnine, an alkaloid drug which is used medicinally in low doses but which is also used as a poison for rats and other vermin. Its constituent atoms have been colour-coded: carbon (C) is dark blue, hydrogen (H) is white, nitrogen (N) is light blue and oxygen (O) is red. Strychnine has a chemical formula of C21H22N2O2, and occurs as colourless or white crystals. It acts as a nerve stimulant or tonic. High doses lead to convulsions, whilst death can be caused by the paralysis of the brain's respiratory centre. It is obtained commercially from plants of the Strychnus genus, especially from Nux vomica seeds.