lactulose use long term

Hello, I am wondering about the effects of long term use of lactulose. My son was prescribed it for long term taking it twice a day, but he was suffering a lot of diahorrea. He was taken off lactulose by the doctor now he is getting a lot of pain and discomfort from constipation. How safe is it to use over the long term.

               Thanks

             Miss. K Melvin