Sven used to be treated with Lactulose for his constipation problems, and it DID work in the beginning, but after awhile it began to lose it's effectiveness, and that's when I began to investigate other alternatives. Our Vet didn't know anything about Miralax until I brought it up. She then investigated it and said it as AOK to use, even though Sven was a CRF kitty.
I much prefer to Miralax because 1) it can be used daily, 1) it's much cheaper, and 3) it continues to work!
But don't be disappointed about the Lactulose. For first time issues, it will probably do the job!