Phase IV

Also known as a postmarketing surveillance trial, is conducted after a drug has received regulatory approval. It involves safety surveillance (pharmacovigilance) and ongoing technical support to ensure long-term safety and effectiveness. The trials are designed to detect rare or long-term adverse effects in a larger population and over a longer period than in Phase I-III trials. Findings from Phase IV trials may lead to a drug being withdrawn from the market or its use being restricted.

Source: Wikipedia