Premarin may have varying side effects classified by severity and occurrence.
Serious but rare side effects are heart attack, stroke, blood clots, dementia, breast cancer, uterine and ovarian cancer, high blood pressure, increased blood sugar, gallbladder disease, liver problems, enlargement of uterine tumors (fibroids), and severe allergic reactions. You should immediately consult your healthcare provider if you observe warning signs such as new breast lumps, unusual vaginal bleeding, changes in vision or speech, sudden severe headaches, or chest/leg pains with or without shortness of breath

Less severe and common side effects include headaches, breast pain, irregular vaginal bleeding, abdominal cramps, nausea, vomiting, hair loss, fluid retention, and vaginal yeast infections.