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Giving a cat a small amount of metamucil (scaled-down from the human dose--so really only 1/3 to 1/2 teasp is enough) every 12-24 hours is acceptable to try to prevent constipation.
It should not be given however, if the cat is already plugged-up and unable to go. The new stool formed is just going to build behind what is already there.
Cats that are already constipated need enema(s), sometimes Subcutaneous fluids, and in svere cases manual (digital) freeing-up and removal of the hard stool at the end of the colon.