FDA Toxicity Grading Scale: Grades, Definitions, and Trial Use

The FDA toxicity grading scale used across clinical trials is the Common Terminology Criteria for Adverse Events (CTCAE), which sorts every side effect into one of five grades: Grade 1 (mild), Grade 2 (moderate), Grade 3 (severe), Grade 4 (life-threatening), and Grade 5 (death). Developed by the National Cancer Institute with input from the FDA, it gives investigators, sponsors, and regulators a shared way to describe how bad an adverse event is, based on how much it disrupts the patient’s daily function and what level of medical care it demands. The current version, CTCAE v6.0, was released in 2025.¹

The Five Grades, Defined

Each grade reflects two things: what the patient can still do for themselves, and what kind of intervention the event requires. The scale leans heavily on a distinction between two categories of daily activity. Instrumental activities of daily living (IADLs) are tasks like grocery shopping, cooking, managing money, and using transportation. Self-care activities of daily living (ADLs) are the more fundamental ones: bathing, dressing, feeding yourself, using the toilet.² The line between Grades 2 and 3 usually falls right on that boundary.

Grade 1 — Mild. No symptoms or only mild ones, often picked up by lab work or a physical exam rather than reported by the patient. No treatment needed.
Grade 2 — Moderate. Symptoms are noticeable. Minimal or noninvasive treatment may be indicated. The patient starts having trouble with IADLs, like shopping or managing medications.
Grade 3 — Severe. Medically significant but not immediately life-threatening. Hospitalization or a longer hospital stay is typically warranted. The patient can no longer handle self-care ADLs like bathing or dressing without help.
Grade 4 — Life-threatening. The event puts the patient at immediate risk of death and requires urgent intervention.
Grade 5 — Death. The adverse event resulted in the patient’s death.

Not every adverse event uses all five grades. Some conditions top out at Grade 3 or Grade 4 because higher grades don’t apply to that particular event. Where a grade isn’t available for a term, the CTCAE marks it with a dash rather than leaving it blank.²

How Grades Attach to Specific Events

The generic grade definitions above are the framework. The actual grading criteria live inside the CTCAE document, which is organized by System Organ Class and gives each individual adverse event its own tailored definitions.³ Two examples show how that plays out in practice.

Symptoms

For clinical symptoms, grades are pinned to functional impact and the treatment required. Diarrhea is graded by how many extra stools a patient has per day and whether they need IV fluids. A few extra trips to the bathroom is Grade 1; needing IV hydration puts it at Grade 3. Hypertension uses specific blood pressure thresholds: a sustained systolic reading of 140–159 mmHg is Grade 1, while 180 mmHg or higher with signs of organ damage is Grade 3.²

Lab Results

Lab-based events use objective numerical thresholds, usually expressed as multiples of the Upper Limit of Normal (ULN). Alanine aminotransferase (ALT), a liver enzyme, illustrates the approach in CTCAE v6.0:

Grade 1: ALT above normal but no higher than 3× ULN
Grade 2: Greater than 3× to 5× ULN
Grade 3: Greater than 5× to 20× ULN
Grade 4: Greater than 20× ULN

When a patient’s baseline ALT is already elevated before treatment starts, the CTCAE uses a different calculation based on the ratio of the new value to that abnormal baseline.² The point of this level of specificity is that a Grade 3 lab finding in a trial run in Tokyo means the same thing as a Grade 3 lab finding in a trial run in Houston.

“Severe” Is Not the Same as “Serious”

The two words sound interchangeable and measure different things. Severity is the CTCAE grade and describes how intense an event is. Seriousness is a separate regulatory classification about outcome.

Under federal regulations, an adverse event qualifies as “serious” if it results in death, a life-threatening situation, hospitalization or a prolonged hospital stay, persistent or significant disability, a birth defect, or any other event that requires medical intervention to prevent one of those outcomes.⁴ A Grade 2 (moderate) allergic reaction treated in the ER can be serious even though it isn’t severe. A Grade 3 (severe) rash that’s painful and disabling but resolves without hospitalization may not meet the regulatory definition of serious. Every adverse event in a trial gets assessed on both tracks.

A Different Scale for Preventive Vaccine Trials

The CTCAE was built with cancer patients in mind, meaning people who may already have symptoms or lab abnormalities at baseline. That makes it a poor fit for healthy volunteers in preventive vaccine trials. For that population the FDA uses a separate guidance document with its own toxicity grading scale.⁵

That vaccine scale uses a four-grade structure (mild, moderate, severe, potentially life-threatening) with thresholds calibrated for people who start out healthy. A fever above 104°F is Grade 4. Local injection-site redness exceeding 10 cm is Grade 3. The FDA recommends sponsors of preventive vaccine trials use those tables rather than the CTCAE.

How Grades Drive Trial Decisions

Grades don’t just document what happened. They trigger decisions about dosing, enrollment, and reporting while the trial is running.

Dose-Limiting Toxicities

In early-phase dose-escalation trials, the central question is how much drug a patient can tolerate. That’s answered through dose-limiting toxicities (DLTs), which are predefined adverse events severe enough to set the ceiling for further dosing. DLT definitions vary by protocol but usually rely on CTCAE grades as the trigger. Common thresholds include any Grade 4 drop in blood cell counts, any Grade 3 or higher non-hematologic toxicity, and Grade 3 febrile neutropenia. Certain milder side effects like brief nausea or short-lived fatigue may be carved out so they don’t artificially limit the dose.

Stopping Rules

Trials also set predefined rules that pause enrollment or shut down the study if a pattern of high-grade toxicities emerges. The FDA has noted that toxicity grading can be used to define these stopping rules, where a specified number of adverse events at a given grade triggers a pause or termination.⁵ A rule that says “stop enrollment if three patients experience Grade 3 hepatotoxicity” only works if every site is grading liver events the same way.

Reporting Deadlines

When an adverse event is serious and unexpected, federal regulations impose strict deadlines. If the reaction is fatal or life-threatening, the sponsor must notify the FDA within 7 calendar days of first learning about it. For other serious and unexpected suspected adverse reactions, the deadline is 15 calendar days. Both deadlines also require the sponsor to notify all investigators in the trial.⁴ Investigators commit to those reporting obligations when they sign FDA Form 1572 before joining a trial.⁶

Alongside the grade, investigators assign an attribution: unrelated, unlikely, possible, probable, or definite. Events rated possible, probable, or definite are treated as having a reasonable causal relationship to the study drug.⁷

Which Version Applies

CTCAE v6.0 is the current version, released in 2025 through a collaboration between NCI’s Division of Cancer Treatment and Diagnosis, NCI’s Division of Cancer Prevention, and the FDA, following an extensive public comment period. Protocols already in progress typically specify which version they’re using and stick with it through the end of the study to avoid mid-trial inconsistencies. To bridge old and new data, NCI publishes a mapping resource that aligns every term-and-grade combination from v5.0 to v6.0.¹ If you’re reading a trial’s safety data, check which version the protocol used before comparing numbers across studies.

1
National Cancer Institute. NCI Releases Updated Version of the Common Terminology Criteria for Adverse Events (CTCAE) Reporting in Clinical Trials
2
National Cancer Institute. Common Terminology Criteria for Adverse Events (CTCAE) v6.0
3
National Cancer Institute (NCI). Common Terminology Criteria for Adverse Events (CTCAE) Version 4.0 Quick Reference
4
eCFR. 21 CFR Part 312 – IND Safety Reporting
5
Food and Drug Administration (FDA). Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials
6
Food and Drug Administration (FDA). Form FDA 1572
7
National Cancer Institute (NCI) DCTD. NCI Guidelines for Investigators: Adverse Event Reporting Requirements