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Not every health claim is backed by the same quality of research. To help you see the difference, we give the main claims in our articles one of four evidence grades.
The four grades
Strong: consistent results from multiple well-designed trials or systematic reviews.
Moderate: some good-quality trials, but limited in number or size.
Limited: small, short, or early studies; results not yet confirmed.
Insufficient: little or no reliable human research; mostly anecdote or marketing.
How we decide on a grade
When we grade a claim, we look at:
- How many studies there are.
- How they were designed. Randomized controlled trials and systematic reviews count for more than single observational studies or case reports.
- How many people took part, and for how long.
- Whether the results agree across studies.
- Whether the research was done in people, not only in cells or animals.
We grade the evidence for a specific claim, not a whole product or ingredient. For example, an ingredient can have moderate evidence for one use and insufficient evidence for another.
What the grades don't mean
"Limited evidence" does not mean "doesn't work." It means the research so far is too small, too short, or too early to be confident either way. Some things with limited evidence may turn out to work well; others won't.
"Works in one study" does not mean "proven." A single study, especially a small one, can show a result by chance, or in a group of people who are very different from you. We want to see results repeated before we call the evidence strong.
A grade is not a personal recommendation. A well-supported treatment may still be the wrong choice for you. Only a doctor or dermatologist who examines you can say what's right for your situation.
When grades change
Research moves on. When new studies are published, we review the affected articles and change the grade if the evidence has changed. Every article shows the date it was last updated.