Severn Trent Water is reassuring residents that a stolen lethal poison would not pose a public health risk if it got into the water supply.
Five 100g pots of potassium cyanide, which is used in gold plating, were taken from a locked store cupboard at premises in Halesfield in Telford, Shropshire, on Tuesday night.
West Mercia Police warned the deadly substance can kill within minutes if ingested.
A force spokeswoman urged anyone finding the containers to contact police immediately.
She said: "The substance is in crystal form and looks similar to rock salt. Even as little as a teaspoonful could be lethal in minutes. The pots are around five inches tall and grey in colour with a white lid. They are clearly labelled with a poisonous symbol."