Coreg (Carvedilol) serious side effects. Call your doctor at once if you have a serious side effect such as: feeling like you might pass out; slow or uneven heartbeats; chest pain, dry cough, wheezing, chest tightness, trouble breathing; feeling short of breath, even with mild exertion; swelling, rapid weight gain; numbness or cold feeling in your hands and feet; loss of bladder control; pale skin, feeling light-headed, rapid heart rate, trouble concentrating; high blood sugar (increased thirst, increased urination, hunger, dry mouth, fruity breath odor, drowsiness, dry skin, blurred vision, weight loss).