There have been several recalls of blood pressure medications following the discovery of chemicals within them that are known carcinogens, that is, chemicals which increase the risk of cancer. Medicinal drugs are, albeit on a much grander scale, made in the same way that one may have performed chemical experiments in high school science class. In other words, the active ingredient in a drug is the result of a chemical reaction in which chemists and their assistants use existing molecules to create helpful medicine.
However, as with most all chemical reactions, there are going to be byproducts which, in this context, are called "impurities." While many of these are harmless, others are potentially toxic or, at best, the subject of concern on the part of the experts.
Manufacturers of drugs have the obligation to rid their products of impurities that could harm the public. Their chemists may do so by one of several routes.
For example, sometimes altering the chemical reaction, say by keeping it at a certain temperature or changing up the order in which the reaction occurs, may prevent impurities. In other cases, chemists may instead choose to tolerate the impurity initially, but remove it down the road in the manufacturing process.
Manufacturers and their chemists are in the best position to figure out how to make sure that their drugs are safe, which would mean that they are free of harmful impurities. In fact, it is their duty to provide the public with helpful, and not harmful, medical products. If a drug manufacturer fails in this obligation, a medical product liability case may be a means of obtaining compensation.