Arsenic used to be administered as a medicine for certain illnesses.
I believe it was given to those infected with syphilis in the days prior to modern antibiotics.
Medical doctors know full well that almost every known substance which can be used as a medicine is a lethal poison; the only difference being the dosage level which is administered.
There is a rule taught to medical doctors that medicine must be administered to patients in the smallest dosage possible which is effective.