How to Distinguish Between an Adverse Drug Reaction (ADR) and an Adverse Event (AE)

Posted by on January 9, 2014

By definition, a pharmaceutical personal injury cases involves individuals who have claimed to suffer a medical injury in temporal proximity to a drug exposure. The individual medical case reports are sometimes written up in the peer-reviewed scientific literature. But more often they only exist as entries in a spontaneous reporting system such as the FDA’s Adverse Event Reporting System (FAERS). In order to assess the relevance of these cases to a product liability action, it is helpful to understand the distinction between a true adverse drug reaction (ADR) and a more generic adverse event (AE).

An adverse event (AE) is defined as “…any untoward medical occurrence in a patient administered a medicinal product and which does not necessarily have to have a causal relationship with this treatment.”

Based on this definition, AEs can therefore be any unfavorable medical sign or symptom that has some temporal association with exposure to a medicinal product. In other words, in order to qualify as an AE, it is not necessary that a healthcare provider make any determination about the causal link between the medical event and the drug exposure. In fact, a medical practitioner would be acting irresponsibly if he failed to report an event as an AE based on his or her determination that the event was causally unrelated to the drug. This is why AEs entered in the FDA Adverse Event Reporting System (FAERS) will include all kinds of seemingly random (sometimes ridiculous) events that clearly had nothing to do with the drug experience (e.g., receiving a gunshot wound, being hit by a bus, etc.).

An adverse drug reaction, on the other hand, is defined (by the same guideline) as: “…all noxious and unintended responses to a medicinal product related to any dose…” Thus, for an ADR, a causal relationship must be at least suspected by the medical practitioner.

The distinction between an AE and an ADR is summed up nicely in Chapter 10 of the classic textbook, Pharmacoepidemiology (authored by Dal Pan, Lindquist, and Gelperin):

“The principal difference between an adverse event and an adverse drug reaction is that a causal relationship is suspected for the latter, but is not required for the former. In this framework, adverse drug reactions are a subset of adverse event reports.”

Some databases contain only ADRs, while other databases contain only AEs. When working up a legal action involving these case reports, it is important to be clear as to which type of event is being posited. If the event is written up as an AE, it is important to be very clear that no causal link was posited by a medical practitioner.

Finally, as we have discussed in previous posts (see here, here, and here), it is critical to note that neither ADRs or AEs are capable of providing adequate evidence for causal inference. Because these events are reported anecdotally and spontaneously and were not collected as part of a systematic protocol, they are, by their very nature, anecdotal and serve as hypothesis-generating signals. They can (and should) play a role in an assessment of your client’s liability (in terms of their conduct and how they followed up on a given signal) in a particular case if causality has been adequately established (using other, more rigorous means). However, these events (AEs or ADRs) should never be allowed to serve as evidence of causation under any circumstance.


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