Just wanted to record our experience with using both lactulose and Miralax for an 8-yo cat with no known health problems or disabilities, but suddenly a few months ago she became chronically constipated. She was given an enema twice and was cleaned out by the vet twice, and she was prescribed lactulose. She was getting 1 ML of lactulose 2x/day. I also transitioned her to canned food with only a little bit of dry for an afternoon treat. It wasn't helping. This went on for over 2 months. During that time our first bottle expired so we were given a new bottle, but it wasn't helping either.
In desperation I tried milk along with the lactulose, 1/8 c of whole milk with meals. She drank the milk OK, but no real change in the litterbox, so 2 pints of milk later, I gave up on that.
By now it had been 3-4 months (not sure how long, I've lost track) and she's had maybe 2 normal movements that whole time. Most days she was having nothing (often for several days), or a very dry deposit maybe 1 1/4" long, but she continued to eat OK. I got more desperate and tried Miralax. We have been doing this for almost a week, and it is giving some results. She is getting 1/4 tsp morning and night. You dissolve it in water and mix the solution into canned food. Apparently it is tasteless or nearly so.
Common advice is that the cat is not drinking enough water and you should encourage drinking. All I can say is, I was cleaning plenty of urine clumps out of the box, so it seems like her fluid intake must be pretty good.
The rest of what I have to say is unscientific, but I'll say it anyway. In years past I have used lactulose with several pets and had a good opinion of it. I have used various brands, whatever the vet had in stock at the time. Sometimes they were colored. One time it smelled like cake mix. The kind we had this time was an unscented, unflavored generic veterinary brand, and as long as it has the main active ingredient you would think it would work.
What I noticed was, the first few times I gave it to her (by syringe) she was not used to it and we dribbled and I got some on my jeans and the floor. Now 3-4 months later, those spots of lactulose are still stuck on my jeans to this very day, and they have been washed probably at least 10 times. If you try to scrape it off with your fingernail, the closest thing I can compare it to is superglue or nail polish spilled on your clothes, it does not scrape off. The jeans have been both washed and put through the dryer multiple times, so maybe the dryer had something to do with it. But it dried the same way on the carpet, and it did not wash off the carpet with water and a scrubber pad. I ended up using alcohol wipes to get it off the carpet after it dried there.
I started wondering, if the purpose of lactulose is to draw more water into the colon so the stools are softer, and this stuff does not even attract enough water in the washing machine to be washed out of denim, what is happening when it stays in her colon for days and weeks and she is not going? Does it turn hard and dry, like it does on jeans and carpet? I don't know. Right now we are doing better with Miralax.