For a variety of reasons and in many ways, doctors and medical researchers often fail to investigate the harms caused by medical treatments.
This is the royal road to over-treatment: it encourages doctors to be overoptimistic on their patients’ behalf. It also skews or makes impossible so-called informed consent: for if the harms are unknown even to the doctor, how can he inform the patient of them? The doctor becomes more a propagandist than informant, and the patient cannot give his informed consent because such consent involves weighing up a known against an unknown.