1. Proof of “but for” aspect: Thus the plaintiff does not have to prove with absolute certainty that had it not been for the defendant’s conduct, the injury would not have occurred. All he has to do is to show that it is probable that the injury would not have occurred without the defendant’s act.
Example: P, a 250 lb. woman, falls down an unlit staircase outside of the waiting room of D Railroad. P argues that D’s failure to light the staircase was the actual cause of her injury; D contends, however, that P might have fallen even had the stairs been brightly lit.
Held, P has adequately established actual cause. “ . . . Where the negligence of the defendant greatly multiplies the chances of accident to the plaintiff, and is of a character naturally leading to its occurrence, the mere possibility that it might have happened without the negligence is not sufficient to break the chain of cause and effect between the negligence and the injury.” Reynolds v. Texas & Pacific Ry. Co., 37 La. Ann. 694 (La. 1885).
a. Inference by jury: In fact, most courts do not actually require P to “prove,” in any exact sense, even that it is more likely than not that D’s negligent act caused the particular damage at issue. Instead, the jury is permitted to make common-sense inferences that the negligence caused the damage, as long as such an inference is not unreasonable. Thus in Reynolds, supra, unless a modern court would conclude that no reasonable jury could believe that it was more probable than not that the unlit staircase was a “but for” cause of the accident, P would be permitted to get the jury.
b. Dosages in medical malpractice cases: An illustration of this principle arises in medical malpractice cases where the claim is that the defendant administered an unsafely-large dose of an otherwise-safe drug. The plaintiff must theoretically prove that the “extra” dose made the difference (i.e., that the recommended dose would not have produced the damage). But the court will be quick to let the jury decide that because dosages are set at particular levels for good scientific reasons, a dose significantly in excess of the recommended level is likely to have caused the damage.
Example: A doctor working for D (the U.S.) negligently prescribes twice the maximum recommended daily dose of the drug Danocrine for P. P takes the too-high dosage every day for the next month. During that month, she develops various symptoms of illness, and is soon thereafter diagnosed with a rare and fatal disease, primary pulmonary hypertension (“PPH”). After P eventually dies from PPH, her estate sues D. At trial, P’s expert medical witness provides good evidence that P’s exposure to the Danocrine caused her PPH. But the expert’s evidence is much less convincing as to whether the overdose, as opposed to a properly-sized dose, caused the disease.
Held, for P. “[W]hen a negative side effect is demonstrated to be the result of a drug, and the drug was wrongly prescribed in an unapproved and excessive dosage (i.e. a strong causal link has been shown), the plaintiff who is injured has generally shown enough to permit the finder of fact to conclude that the excessive dosage was a substantial factor in producing the harm.” Therefore, P’s estate has adequately proved that the excessive dosage was the cause in fact of her death. Zuchowicz v. U.S., 140 F.3d 381 (2d Cir. 1998).