The Laxatone will help with the constipation but not the crystals. The double dosing of antibiotics is past history so there is no use to worry about it now. I have used the Carpon in my cat and it seems to help. The vet I worked for for over 20 years does not believe in it but he is "from the old school". It isn't going to hurt anything. Keep his litter box as clean as possible and don't use the really dusty litter.
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