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phase IV trial
med.	Testing in humans that occurs after a drug or other treatment has already been approved by the Food and Drug Administration FDA and is being marketed for sale. Phase IV trials are conducted to determine long-term safety and effectiveness and to identify adverse effects that may not have been apparent in prior trials. Thousands of participants are usually recruited to volunteer for this phase of clinical testing. see also clinical trial, food and drug administration, phase I trial, phase II trial, phase III trial