Any drug can conceivably have undesired side effects or Adverse Drug Reactions (ADRs). During drug development, clinical trials are used to assess the efficacy and safety of a drug. The spontaneous post-marketing reporting system helps ensure that any additional ADRs continue to be reported to the FDA and other World Wide Regulatory Authorities and to the manufacturer after the drug is marketed.

ADRs associated with the use of a drug can lead to hospitalization, disability, life threatening reactions, and even death. Although the United States has a rigorous drug approval processes, all potential drug safety issues cannot be detected during the clinical trials of a drug. When a drug is first approved for marketing in the United States, the initial package insert only reflects what is known about the drug at that time. Clinical trials are of short duration, often exclude young and older individuals along with those individuals taking other medicines, or with other medical problems and complications. Once a drug is on the market, and thousands or even millions of patients are exposed to it, new safety information will continue to evolve over the lifetime of the drug, with ongoing updates of the official labeling needed to incorporate new safety-related information. The majority of "new" adverse drug reactions are detected during a product's first few years on the market, but clinically significant and serious safety issues can arise even when a drug has a marketing history of over many years. How a pharmaceutical company handles adverse reactions and updates their labeling is critical in product litigation.