My cat was lethargic, drinking extra water, and his coat lacked shine. I had great luck using powdered ascorbic acid (vitamin C) mixed with water for this condition. I put a half teaspoon into a small cup, added water, then used a plastic syringe (available at drugstores) to give to my cat orally.
The next morning after giving vitamin C, he was plumper, healthier, and happy. I will continue to give him 250 mg a day for now to take care of any problems he mght be experiencing with his kidneys. Apparently the max dose is 250 a day for cats.
I was getting worried about my kitty and am so happy to see him improving so rapidly! High dose vitamin C therapy prevents all kinds of human diseases as well!