German cancer experts are warning of a looming shortage of one of the most widely used drugs to treat the disease—because pharmaceutical companies say it is not financially worth their while to make it any more. Fluorouracil, also known as 5-FU, is a chemotherapy agent used to treat bowel and breast cancer. It is one of the most commonly prescribed drugs for cancer patients worldwide. But many of Germany’s top providers have stopped making the drug because it is not as profitable as other, more expensive ones. Of the six main companies which supplied 5-FU to pharmacies across the country,...