Disclosed is a method of treating or preventing mastitis in a non-human animal, said method comprises the step of applying a medicament to the mammary gland of the non-human animal wherein the medicament contains an extended acting oxygen generating system which has at least two components: (i) one component being a slow release hydrogen peroxide generation system, and (ii) another component being an activator in the form of a halide or thiocyanate substrate wherein the extended acting oxygen generating system is configured to provide oxygen generation over a time period between 30 seconds to 14 hours following administration to an animal, and the two or more components present in the medicament are in a form prior to administration to an animal that prevents oxygen generation until during or after administration and controlled thereafter.
