According to the National Toxicology Program website of the Department of Health and Human Services potassium ferricyanide is not particulary toxic. They mention a woman who ingested a coffee spoonful or the solid and did not suffer any serious effect. The cyanide ligands seem to be very tightly bound to the iron atom. This chemical was in my first chemistry set. The MSDS does mention that excposure to high heat can release hydrogen cyanide as well as strong acids.