I had a question regarding rates of occurence in FAERS. According to the FAERS documentation,

Rates of occurrence cannot be established with reports: The number of suspected reactions in FAERS should not be used to determine the likelihood of a side effect occurring. The FDA does not receive reports for every adverse event or medication error that occurs with a product. Many factors can influence whether an event will be reported, such as the time a product has been marketed and publicity about an event. Therefore, information in these reports cannot be used to estimate the incidence (occurrence rates) of the reactions reported.

In light of this, is there a way to directly compare two different drugs that would account for total prescriptions? I've reviewed several publications based on the FAERS data which seem to use ROR and PRR for the purpose of comparing across drugs; however, neither of these methods account for total prescriptions.

Furthermore, to establish a rate (e.g., # reports of AE XX/100K prescriptions), I anticipate that it would difficult, if not imposssible, to identify an appropriate denominator, so it's unlikely that the results would even be meaningful/interpretable. There are likely other limitations of the FAERS system (e.g., data quality, duplicate reports, incomplete reports, etc.) that would further complicate this task.

FWIW, we are looking specifically at PTs related to abuse (e.g., drug abuse, drug dependence, and drug withdrawal syndrome).

Many thanks in advance for your any assistance that you are able to provide.

1 Answer 1


Your answer is essentially found in the quoted text. The information in FAERS is incomplete, containing only the events actually reported to the FDA. You can determine the number of events reported, but even that doesn't indicate total prescriptions in any way.

Sorry not to be of more assistance.

Violet Wren, openFDA Tech Lead

  • Thank you for the reply, Violet. It is most greatly appreciated. Just so that I am clear, even if you had total prescription data, sourced from a seperate database. You still could not reliably fold those into the FAERS data to determine a rate for the numerous reason that FDA notes in their explanation provided above.
    – Steve
    Commented May 19, 2023 at 12:24

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