A simple guide for understanding two very different conditions Many people experience symptoms before their period, but not all premenstrual symptoms are the same. PMS and PMDD are often confused, but they differ in severity, impact, and how much they affect daily life . Here's a breakdown to help you better understand the differences. Quick Comparison Table Feature PMS (Premenstrual Syndrome) PMDD (Premenstrual Dysphoric Disorder) How common? Very common (up to 90%) Less com