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The dosage range for cephalexin (KeflexÂ®) is most often 22 to 33 mg/kg. Great Danes are big dogs as you know (weight range can be 110 to 160 pounds). Let's say your dog weighs a modest 110 pounds. They would be roughly 55 kg. 55 kg x 22 mg/kg = 1210 mg or if they used the 33 mg/kg = 1815 mg.
Your dog should not have any side effects from the Vitamin K, but it may not help forsure. Keep giving it to him though, as your Vet recommended because that is the issue right now.