Before this poison issue gets too far, I'd like to try to clarify.
All medications, and many other things, affect the way the body is working. One takes them because, presumably, the body (or parts of it) are not working correctly. The patient takes them, hopefully well informed, in order to fight DISease (emphasis is mine). This is the benefit. The cost is the possibility of adverse reactions, and unpleasant side effects (not to mention having to go to the doctor and possibly spend a lot of money).
As an illustration, imagine a young infant whose body is functioning perfectly. Then, it gets pneumonia. The doctor prescribes an antibiotic, which kills the bacteria and allows the lungs to heal. Unfortunately, the child develops diarrhea as a result of the antibiotic.
Fortunately, the diarrhea stops after the antibiotic is finished. The child goes back to being perfect.
One could say that the child was poisoned by the antibiotic, but most would agree that it was worth it given the favorable outcome, and the potential for a bad result from the pneumonia (like death) if treatment wasn't given.
It's the same with every medication. Some, used for very serious problems, or problems that are not well understood (like PN) may be more "poisonous" and less beneficial.
Remember, many people purposely put poisons into their bodies, which have no real benefit (other than maybe feeling good), like alcohol,
tobacco, illegal drugs, massive amounts of sugar, etc.