A defective drug is one that causes undisclosed, severe side effects. A drug can be defective because its maker failed to warn patients about the risk or because it contains the wrong ingredients.
Examples of defective drugs for which pharmaceutical companies failed to issue adequate warning include Actos (pioglitazone for diabetes, which causes heart failure and bladder cancer) and Pradaxa (dabigatran for breaking up blood clots, which causes life-threating internal bleeding).
Any drug product can have manufacturing defects at any time. Analyzing doses from the batch including the drug used by the patient who suffered a severe, unexpected side effect will be necessary to identify the error.