Lactulose generally takes about 24 to 48 hours (up to 2 days) to start working to relieve constipation. It does not act immediately and may take a couple of days to soften stools enough to enable comfortable bowel movements. If constipation persists beyond 3 to 5 days of taking lactulose, it is advisable to consult a doctor. Lactulose works as an osmotic laxative by drawing water into the bowel to soften stool and facilitate passage.