Some side effects that can occur include:
stomach upset or pain, nausea, vomiting, bloating, breast tenderness/pain/swelling, headache, weight or appetite changes, freckles or darkening of facial skin, acne, increased hair growth, loss of scalp hair, problems with contact lenses, vaginal itching or discharge, changes in your menstrual periods, decreased sex drive, nervousness, dizziness, or, tired feeling
Some interactions may occur if you are taking St. John's wort, carbamazepine, phenobarbital, rifampin, ritonavir, ketoconazole, itraconazole, and some antibiotics with Prempro.
If you experience any side effects that become severe, contact your doctor for additional treatment options to help manage the side effects.