Background: Medium-chain triglyceride (MCT) enriched diet has a positive effect on seizure control and behavior in some dogs with idiopathic epilepsy (IE).Objective: To evaluate the short-term efficacy of MCTs administered as an add-on dietary supplement (DS) to a variable base diet to assess seizure control and antiseizure drug's (ASD) adverse effect profiles.Animals: Twenty-eight dogs with International Veterinary Epilepsy Task Force Tier II (IVETF) level diagnosis of treated IE with 3 or more seizures in the last 3 months were used.Methods: A 6-month multicenter, prospective, randomized, double-blinded, placebocontrolled crossover trial was completed, comparing an MCT-DS with a control-DS.A 9% metabolic energy-based amount of MCT or control oil was supplemented to the dogs' diet for 3 months, followed by a control oil or MCT for another 3 months, respectively. Dogs enrolled in this study satisfied most requirements of IE diagnosis stated by the IVETF II level. If they received an oil DS or drugs that could influence the metabolism of the investigated DS or chronic ASD, the chronic ASD medication was adjusted, or other causes of epilepsy were found, the dogs were excluded from the study.Abbreviations: ASD, antiseizure drug; BHB, beta hydroxybutyric acid; C10, decanoic acid = capric acid; C8, octanoic acid = caprylic acid; DS, dietary supplement; IE, idiopathic epilepsy; ITTA, intention to treat analysis; IVETF II, International Veterinary Epilepsy Task Force Tier II; KBr, potassium bromide; LEV, levetiracetam; MBW, metabolic body weight; MCT, medium-chain triacylglyceride; ME, metabolic energy; PB, phenobarbital; QoL, quality of life; RER, resting energy requirement; RVC, Royal Veterinary College; VAS, visual analog score.