By MedClinRes.org Clinical Research Team
A study's adverse event table can look like a verdict on safety. It isn't one. That table is a record of what happened to a specific group of trial participants, under specific monitoring conditions, over a specific period of time — and reading it well means understanding what those numbers do and don't say before drawing any conclusion about a particular medicine. This guide walks through how to read an adverse event (also called a “harms” or “safety”) table in a clinical trial report, using the reporting standards researchers are expected to follow and the FDA's own public adverse-event data as reference points, without translating any of it into a personal safety judgment.
What an Adverse Event Table Actually Shows
Most randomized trial reports include a table listing adverse events by treatment group, usually with a count and percentage of participants who experienced each event. That structure looks simple, but each column carries assumptions that change what a number means:
- Event definition — what counts as “headache,” “nausea,” or “serious adverse event” is set by the trial's own protocol and coded against a standard terminology (commonly MedDRA). Two trials can use different thresholds for the same word.
- Group size (the denominator) — a rate only means something next to the number of people who could have had the event. “12 cases” is a different finding in a trial of 40 participants than in a trial of 4,000.
- Severity — trials generally grade events as mild, moderate, or severe, and separately flag “serious adverse events” — a regulatory term defined by specific outcomes such as death, hospitalization, or a life-threatening event, not by how unpleasant an event felt.
- Timing — when an event occurred relative to starting treatment, and how long participants were followed, shapes whether an event could plausibly relate to the drug at all.
- Discontinuation — the number of participants who stopped treatment because of an adverse event is often a more informative figure than the raw event count, since it reflects tolerability in practice.
The CONSORT Statement (CONsolidated Standards Of Reporting Trials) is the reporting framework most medical journals ask randomized-trial authors to follow. It is, in its own description, “an evidence-based, minimum set of recommendations for reporting the results of randomised trials,” built around a checklist and a participant flow diagram so readers “have all the information they need to critically appraise, interpret and use the research.”
Why Harms Reporting Gets Its Own Standard
Benefit outcomes in a trial are usually prespecified precisely. Harms reporting has historically been looser — which is why CONSORT has a dedicated extension for it. The 2023 update, CONSORT Harms 2022, revises 13 checklist items and adds three new ones specifically for how trials should report adverse events. Among its requirements: authors should state whether harms were collected systematically (structured questionnaires or clinical exams) or passively (spontaneous reporting), describe how and when harms were assessed, report any methods used to judge whether an event was related to treatment and who made that judgment, and report zero-event outcomes rather than omitting them — a step aimed at preventing selective reporting. The guideline's authors cite prior research on underreporting of serious harms — including deaths — in published trial results as part of the rationale for these changes, which is the kind of gap a table-reading checklist can't fix but should make a reader cautious about.
In practice, that means a table with cleaner, better-defined categories and a stated collection method deserves more confidence than one with vague event labels and no explanation of how harms were tracked.
What FDA Adverse Event Data Adds — and What It Doesn't
Trial safety tables aren't the only source of adverse event information. Once a drug is on the market, the FDA collects postmarket reports through what was long known as FAERS (the FDA Adverse Event Reporting System). In 2026, FDA consolidated FAERS and several other legacy safety databases into a single public platform, the Adverse Event Monitoring System (AEMS), which the agency says is intended to make postmarket safety data available in closer to real time. The underlying caution that applied to FAERS data applies to AEMS data as well, and FDA states it directly: “The existence of adverse event reports for a drug or biologic…does not mean that the drug or biologic caused the adverse event.” Reports reflect what a patient, caregiver, clinician, or manufacturer chose to submit and describe — not a confirmed medical determination.
That distinction matters because postmarket reporting has structural limits that a trial's controlled reporting doesn't have to the same degree:
- Reporting is largely voluntary for patients and healthcare professionals, so the database is understood to capture only a fraction of events that actually occur.
- There's no reliable denominator — the system doesn't know how many people took a drug without incident, so raw report counts can't be turned into a rate.
- The same case can appear more than once (for example, reported separately by a consumer and by the manufacturer), and a single case can list multiple reactions, so summed counts can look larger than the number of distinct people affected.
- FDA's own guidance is explicit that this data “by themselves are not an indicator of the safety profile of the drug.”
The practical takeaway: postmarket adverse event data is useful for spotting patterns FDA may want to investigate further, and it's a public window into what's being reported about a drug. It is not, on its own, a safety determination — for the same reason a trial's raw event count isn't one either.
A Short Checklist for Reading Any Safety Table
Before drawing any conclusion from a table, it helps to locate six things:
- How is each event defined, and was a standard terminology used?
- What is the group size behind each percentage — and is it the same denominator throughout the table?
- How is severity graded, and is “serious adverse event” being used in its regulatory sense or its plain-language sense?
- When did events occur relative to starting treatment, and how long was follow-up?
- How many participants discontinued treatment because of an adverse event, separate from how many merely reported one?
- Was harms collection systematic or passive, and does the report say so?
If a table or its surrounding text doesn't answer one of these, that's a gap worth noting rather than filling in with an assumption.
Questions Worth Bringing to a Clinician or Pharmacist
A safety table describes a study population, not any one reader. Translating it into a personal decision is exactly the step this guide doesn't attempt — that's a conversation for a licensed clinician or pharmacist who knows an individual's health history, other medications, and risk factors. Useful questions to bring to that conversation include: How does my own health history compare to the population studied in this trial? Are any of the reported adverse events relevant to conditions or medications I already have? Was the discontinuation rate in this trial higher or lower than what's typical for similar treatments? Is there more recent postmarket safety information, such as an FDA safety communication, that updates what this trial found?
What This Guide Does Not Do
This article explains how to read the structure of a safety table and where FDA's public adverse event data fits alongside it. It does not evaluate any specific drug, supplement, or treatment, and it is not a substitute for reading a trial's full published methods or for guidance from a licensed healthcare provider. Readers with questions about whether a particular medicine is appropriate for them should discuss the full clinical picture with a clinician or pharmacist rather than relying on a single table or database entry.
For related reading on how to evaluate trial evidence, see our guides on how to find a trial's primary outcome and time frame, how to match a study with its registry record, and why the studies included in a systematic review matter. For help interpreting the results a trial reports alongside its harms, see what a confidence interval adds to a result. See also our Editorial Standards and Medical Disclaimer.