My elderly cat suffered from extreme constipation. She would strain and nothing would pass. I found two things that worked well. Canned pumpkin - 1/2 teaspoon each day. It takes a couple of days to work through the system, but it did the trick. Also, my holistic vet recommended slippery elm. I opened a capsule and put about 1/4 capsule in some liquid, dissolved it, then gave it in a syringe. This also worked very well.
Our cat has been constipated for almost two years. He has had several enemas, has been on lactulose, powdered laxative, wet food, etc. The last time we brought him we thought we would have to say goodbye, it was so bad. I finally looked up constipated cat on line and headed to Vitamin Cottage and got him oat bran, organic pumpkin and organic coconut oil. I mix the coconut oil with all natural wet cat food (with flax & fiber) and organic pumpkin, he loves it! He has gained weight, his coat looks great, his eyes are brighter and he is himself again. We bought him a kitty fountain as well and put water with electrolytes & ice cubes when it's hot, anything to get him to drink and soak up the water. He was doing so well that just this week we slacked on giving him the lactulose and he is backed up again, so keep that up with the new diet. He loves coconut oil, he eats it like ice cream!