Cesium has been shown to have a pronounced physiological action on animals when administrated in amounts equivalent to potassium. Hyper-irritability including spasms were found eventually leading to death. Cesium metal can cause serious burns when it comes into contact with the skin. Its extreme reactivity with moisture and oxygen requires it to be handled with extreme care.

The toxicity of cesium compounds is generally of very minor importance except when cesium is combined with another toxic metal or radical. Such an example would be cesium cyanide or cesium gallium sulfate.

Cesium-137 is used in radiation treatments, for instance in cancer therapy.