This phase is also known as post-marketing surveillance: it concerns the monitoring of authorized drugs after they have been placed on the market.  

Phase IV studies are conducted to gather additional information on the long-term safety and efficacy of the drug, when used under real-life conditions and by patients with various concomitant diseases.  

They are conducted in large groups of more than 1,000 patients and are used to detect rarer side effects and side effects with long-term use, to better identify interactions with other drugs and treatments and to optimize treatment.  

These post-approval safety and efficacy studies may be conducted voluntarily or mandated by regulatory authorities. Phase IV studies also evaluate aspects such as quality of life and cost-effectiveness. 

Phase IV clinical trials may be randomized or non-randomized, blinded, double-blind or open-label, with or without a control group.  Because clinical trials in this phase study long-term effects, it is not surprising that they typically last at least several years.