mitral valve prolapse
"I am 39 and identified as having Mitral Valve Prolapse at 28 after worrisome irregular heartbeat manifested itself typically in the evenings. Have used atenolol since then at low dosage (12.5-25mg/day) and am thrilled with the results.  Have experienced no side effects but for a little lethargy upon first starting out (either has passed or I've grown accustomed to it in the evenings)."