In a draft revision of an April 2011 guidance, the FDA has laid out its current thinking on when the agency may require postmarket studies or clinical trials for approved prescription drugs.

The draft adds new information on when the FDA’s Adverse Event Reporting System (FAERS) and Vaccine Adverse Event Reporting System (VAERS) may be sufficient for identifying and assessing new serious adverse drug events that “occur rarely and are closely linked in time to the initiation of the drug and for which the background rate of events is low,” the draft says.

Certain questions related to serious risk “may only be answerable through specific types of studies or clinical trials,” the agency says. For example, animal studies or clinical pharmacokinetic and pharmacodynamic trials may be the only way to determine if a drug is carcinogenic or could potentially interact with other treatments.