Chloramphenicol is an approved drug, but not approved for this purpose. Approving Chloramphenicol as a coronary treatment requires human trials that will probably cost $25 million.
I am extremely far from an expert here so there may be some subtlety, but off-label uses are generally possible. From Wikipedia:
However, once a drug has been approved for sale for one purpose, physicians are free to prescribe it for any other purpose that in their professional judgment is both safe and effective, and are not limited to official, FDA-approved indications. This off-label prescribing is most commonly done with older, generic medications that have found new uses but have not had the formal (and often costly) applications and studies required by the FDA to formally approve the drug for these new indications.
Edit: The full post at the link acknowledges this:
As an approved drug (though for another purpose) any doctor can prescribe Chloramphenicol for any purpose. Of course, they don’t know to do this. And — perhaps more importantly — such bold action can get American doctors sued for malpractice.
I am extremely far from an expert here so there may be some subtlety, but off-label uses are generally possible. From Wikipedia:
Edit: The full post at the link acknowledges this: