Acute and severe reactions after administration of a compounded extended‐release injectable omeprazole in two horses

Summary Extended‐release injectable omeprazole (ERIO) is increasingly used for the treatment of equine gastric ulcer syndrome (EGUS). Reported side effects are mostly self‐limiting local reactions. This report describes two horses with severe reactions. A 7‐year‐old Warmblood mare was diagnosed with EGUS by gastroscopy. Because of difficulties administering oral medication, a compounded ERIO formulation was prescribed. The first injection, by a veterinarian, was uneventful. The owner injected the second dose, after which the mare immediately collapsed. When the veterinarian arrived, the mare was standing but ataxic with a slow pupillary light reflex, tachycardia, tachypnoea and hyperthermia. She was fully recovered 12 h postinjection. A 17‐year‐old Warmblood gelding suspected of having EGUS received ERIO for compliance reasons. During his fourth administration by the owner, the gelding collapsed and convulsed, dying before the veterinarian arrived. No postmortem examination was conducted. Product analysis did not lead to a plausible explanation. This is the first report of severe reactions following administration of ERIO. This highlights the potential for previously unreported side effects and the need for informed owner consent when using unregistered medications that have not completed the same safety studies required for registration.