Overdose symptoms might include lethargy, disorder of respiratory drive, wheezing, slow heart rate and irregular heart rhythm, symptoms of congestive heart failure, low blood pressure and low blood sugar. The usual maxium theraputic dose is 50 to 100 mg daily, though some patients may need up to 200 mg daily.
8 pills is an overdose. You should call the doctor and go to ER.
For more information, try this link: Tenormin (Atenolol Tablets) Drug Information: Uses, Side Effects, Drug Interactions and Warnings at RxList