Digoxin
"I have suffered from tachycardia for 25 years. I was taking Atenolol 25mg for years until it did not really work anymore. My Harley street heart surgeon told me to take also digoxin 250mg. 
I noticed right away my heart was beating more regularly, more stable. After two weeks I was thinking was I supposed also take the Atenolol pills. And started also taking them. That was not good, I was back to tachycardia.
I went back to see my doctor and told him that it was strange because first I thought that the new medication was working and then it did not work anymore. I asked if I have to take also the old medication Atenolol and he told me "YES". After some time I decided to give up Atenolol and Digoxin works better alone !!!"