What is the test for establishing that a drug has caused a person to develop cancer?

The courts reasoning was sound. In Jones v. Ortho Pharmaceutical Corp. (1985) 163 Cal.App.3d 396, 209 Cal.Rptr. 456, this court affirmed a nonsuit ruling where the only evidence that the defendants drug had caused the plaintiffs precancerous condition was "conjectural and ambiguous" expert testimony that the drug "may have had some effect on the development or progression of the disease." ( Id. at p. 402, 209 Cal.Rptr. 456.) The court explained that "[t]here can be many possible causes, indeed, an infinite number of circumstances which can produce an injury or disease. A possible cause only becomes probable when, in the absence of other reasonable causal explanations, it becomes more likely than not that the injury was a result of its action. This is the outer limit of inference upon which an issue may be submitted to the jury." ( Id. at p. 403, 209 Cal.Rptr. 456.)