A bolus for administration to a ruminant by deposition into its rumen, the bolus having a construction including: a) a casing b) a core inside the casing wherein the core is formed from at least two tablets and, characterised in that a permanently amorphous substance at least partially coats one or more of the tablet or tablets. The permanently amorphous substance may be an oil based substance such as petroleum jelly or liquid paraffin. The casing may be coated with a water impermeable material. The casing may be of metal alloy, with the tablets of the core containing electrically conductive material lower in the electrochemical series than the constituents of the metal alloy casing.
