The four Ds of medical malpractice are duty, dereliction (negligence or deviation from the standard of care), damages, and direct cause. Each of these four elements must be proved to have been present, based on a preponderance of the evidence, for malpractice to be found. The principles of psychopharmacology and the information in the package insert for a drug often play a central role in deciding whether dereliction and direct cause for damages were or were not applicable in a particular case. The author uses data from two cases in which patients were inadvertently fatally poisoned by medication to illustrate two ways in which such information can affect the outcome. In one case, the clinician should have known that he was giving a toxic dose to the patient, whereas that was not true in the other case.