Hi. I Am A Writer. In My Story I Need

Hi.
I am a writer.
In my story i need to murder some rule by another. But i don't know about drugs enough.
So
If in my story
I want to kill somebody with heart patient , what kind of drugs can help me ?
Drugs with lowest lethal dose is very good option in my story.
Becuase my murderer must solve drugs in some drink like tea !
In my story problem is drink is Transparent . Then my murderer can't use tablets. Becuase they are white !!!