The FDA released final guidance on how the agency will weigh the legally required factors when considering whether to require a risk evaluation mitigation strategy for medicines.

The agency says it wants to be as flexible as possible without compromising public safety as it implements the Prescription Drug User Fee Act of 2017. Thursday’s guidance finalizes a draft document first published in 2016.

If a drug’s side effects are reversible or preventable, for instance, regulators may require a REMS that ensures that prevention or counter-measures can be used quickly. But drugs that might lead to permanent damage will probably only have a good risk-benefit analysis if there’s a REMS that limits drug exposure “and the associated occurrence of the adverse event.”

Regulators will also consider how frequently a given side effect may occur. “While a high frequency of adverse events may necessitate a REMS to mitigate this risk,” the document states, “FDA may also require a REMS for an infrequent adverse event, if the adverse event is particularly severe.”