What the letter on an answer means.
A grade describes the evidence behind an answer, not how well the answer reads. Here is how the letter is arrived at.
What a grade means
A High
Well-run trials that agree with each other. You can lean on it.
B Moderate
Good studies with limits in size or design. Read them before you act.
C Low
Small, early or mixed studies. A reason to look closer, not a plan.
D Very low
Thin or conflicting. Treat it as a hint and nothing more.
Evidence too thin
The literature can’t support an answer, so the engine refuses and gives the reason.
Refused
A safety rule fired. A personal dose for a compound with no label dosing is never given, whatever the studies say.
The scale is GRADE, the one behind the clinical guidelines you already read. It is applied here to a compound and an indication, on the studies retrieved for it.
How the letter is worked out
Weigh the studies by design
Each study counts by what kind of study it is. Systematic reviews, meta-analyses and randomised trials count most, cohorts less, animal and lab work very little. An A needs a large body of evidence that includes randomised trials and a review that pools them. A B needs at least one controlled study in people. Evidence that comes only from animals or the lab is a D, however much of it there is.
Hold it at C without evidence we can cite in full
A grade above C has to rest on studies whose full text we can cite, or on a substantial body of trials with a pooled review. A pile of abstracts on its own stays at C.
Take off one level at most
If the trials together enrolled too few people, or the pooled results disagree with each other, the grade drops one level. It never drops more than one, so a body that qualifies for A with one of these lands at B. Indirectness is counted only when the evidence comes from animals or the lab alone. Risk of bias and publication bias are not assessed yet, and nothing raises a grade.
Then the verdict
The letter sets the verdict: A adopt, B promising, C watch, D insufficient evidence. Three things can hold the verdict lower while the letter stays. A serious signal in the adverse-event reports for a compound as a whole makes it avoid or watch, depending on how strong the signal is. A compound no regulator is known to have approved is held at promising. Where landmark trials found harm or no benefit for a use, the verdict for that use is held at watch. A grade for a single indication shows the adverse-event signal beside it but takes its verdict from the evidence, because a count of reports cannot say a drug should be avoided for one use. Label warnings are shown with the answer and do not change the verdict. When nothing is retrieved, the result is D, insufficient evidence.
One grade per compound and indication
The letter is worked out once for the compound and the question asked, then carried by every claim in the answer, so claim grades and the overall grade agree. Where a curated grade already exists for that pair it is served directly instead of being re-derived, and the row says which indication the letter is about.
Reading a graded answer
A provisional grade
Most grades are marked provisional, drawn with a dashed edge in the app. It means the letter came out of the rubric and no clinical advisor has signed it off yet. It is a statement about our review process, not about the studies: a provisional A is still an A. The mark clears when a signoff is recorded against that compound and indication.
A refusal is not a D
D says the evidence is thin and names it. A refusal says the retrieved literature cannot support a safe answer at all, so the engine refuses and gives the reason. It does the same when a claim fails the check against its own citation. An interaction the labels do not describe is reported as no data, never as safe.
What a grade is not
It is not a recommendation, not a safety rating and not a score for the compound. An A answer can say a compound clearly does not work. The letter describes how far the literature settles the question, and nothing else. Safety is carried separately, from the drug labels and the adverse-event reports.
Check it yourself
Every claim links to the record it came from. The grade is our reading of that evidence and the citation is there so you can disagree with it. If you do, send us the query id shown with the answer and we can reconstruct exactly what was retrieved and why it scored the way it did.
Report a grade that looks wrongThe rest of the method
Where the sources come from, how every claim is checked against its citation before an answer ships and which questions the engine hands back to you.