Depo-Medrol Injection for Dogs and Horses is used to alleviate pain and lameness associated with osteoarthritis and arthritic conditions in dogs and horses. Depo-Medrol is also beneficial in relieving allergic dermatitis and other allergy related skin conditions in dogs and cats. Each ml of Depo-Medrol contains 20 mg/ml methylprednisolone acetate. Depo-Medrol is given by intramuscular injection. The average dosage for dogs is 20 mg IM, with a range of 2 mg – 40 mg in accordance with the size of dog and severity of condition. The average dosage for cats is 10 mg, with a range up to 20 mg. The usual dosage for horses is 200 mg IM, repeated as necessary. Following intramuscular injection of methylprednisolone acetate, a prolonged systemic effect results. Methylprednisolone acetate.