The answer is: The issue of endangering patients health with an unknown experimental treatment
Experimental treatment is still in the testing stat and still hasn't uncover all the possibilities that the treatment can cause to the patient.
Depending on the dosage and the genetic composition of the patients, experimental treatment cause a lethal effect to the patient. Because of this, the Scientific community require all experimental treatment shall be done to animals before it used to human patients.
When the patients have no choice for other treatments beside the experimental one, the researchers need to openly mention the possible negative effects to the patients.