I believe that the lactulose reduces the ammonia
in your gut and brain and that's why it is important to take, if the ammonia
gets too high it can cause you to have problems thinking, driving, cooking etc. I am not as good as explaining things as Hector so I have taken one of his explanations and am pasting it here.
I also just read an explanation that the lactulose helps your body do the job the liver would do if it was working right. I hope this helps, I found it in the cirrhosis forum.