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Okay thank you for the additional information. Has he had any labwork to check his phenobarbitol levels in his blood since he has been on the medication? If he is having a side effect to the medication his dose may need to be reduced. Checking his phenobarbitol levels may help determine that. But in most cases dogs on phenobarbitol experience side effects with the drug early on. I would suspect that something else may be going on. How was his heart determined to be enlarged? Was it by x-rays?
Okay since his levels were normal then I would think it would be less likely a side effect of the medication. With his panting and his restlessness I would suspect something is making him uncomfortable. I would start off with a physical exam by your veterinarian to rule out any possibility of pain. Dogs that are overweight can experience some joint and arthritic issue so it is definitely a possibility. Otherwise you may want to revisit the enlarge heart issue to make sure nothing changed since 4 months ago