Have been taking lactulose every day for the past couple of months - was prescibed and is perfectly ok during pg. No your body will not become dependant on it, normal service resumes after with no problems as long as you remain hydrated and well fibred up.
lactulose is fine. i dont know how far through you are, but i found constipation eased off towards the end of the first trimester. im now about 25 weeks and drink as much water as i can and eat a tonne of fruit (always starving, try and eat fruit first and dont turn to chocolate until im desperate!). Im also veggie, goodness know what happened to all my poop in the first trimester!!!