Hydrochlorothiazide / triamterene
"I have been on atenolol for years for fast heart rate.  But as I've gotten older, my blood pressure as increases.  My doctor added  triamterene along side of the atenolol.  The first day, I didn't feel well, but assumed I would feel a little different with new meds.  I made sure I drank lots of water.  Days 2-4 days were horrible.  Extremely weak, heart palpitations, chest pain, and overall bad feelings. Went back to only the atenolol.  Today is 2 days after stopping and I feel better as far as the weakness, but my pressure is back up.  Waiting for the doctor to give me something else to try."