I was just reading what some other people posted and someone said side affects are low--that's not the case for everyone! There are TONS of side affects and that person must have been lucky enough to NOT experience them! I had a miscarriage in December and w/in 3 weeks was PG again, so they gave me prometrium to prevent miscarriage again. I took it for 10 PAINFUL weeks. My side affects were severe: nausea (to the point I didn't eat), constipation (so bad, I CRIED), headaches (migraine strength), dizziness/blurred vision, irritablity (my husband's favorite!), and extreme fatigue. Yes, those can be symptoms of first trimester, however, this was my 5th pregnancy and I had NEVER had any of those symptoms, much less to the degree they were. (Plus, my symptoms ALL went away w/in a couple of days of stopping the treatments) I was SOO MISERABLE. The only bonus was that I now have a wonderful 6 week old daughter! ;) My symptoms were so bad that I was unable to function and care for my family/children. They started within a few days of taking the pills. Read the warning labels/packet carefully. They are hormones and too many hormones can cause problems. Good luck :)