Many new drugs have potential the to have adverse drug effects that are not known at the time of launch, so the need for its PM surveillance becomes essential.
While classical postmarketing surveilance has some weaknesses related to the inability to determine the prevalence of an ADR the spontaneous reporting system is essential for discovering rare ADRs and ADRs in specific subpopulations. If you enrich this with active surveillance as is now more common for new drugs even more information can be made available. The three big international databases of spontaneous reports (FAERS/AERS, EV and the WHO (can't remember the name)) are all available for researchers in one way or another. This is a really exiting field where there are many research opportunities.