Drug Synergy Calculator — Bliss Independence Score
Free drug synergy calculator. Scores Bliss independence across a multi-dose combination matrix and flags synergy or antagonism well by well. Runs in your browser.
How it works
Combination effects are scored by Bliss independence, which predicts the response of two non-interacting drugs as E_expected = E_A + E_B − (E_A × E_B). The excess of observed over expected inhibition is the Bliss score: positive indicates synergy, near zero indicates independence, negative indicates antagonism. Scores are computed across every pair in the dose matrix.
Frequently asked questions
What is a good Bliss synergy score?
A Bliss score is the observed effect minus the effect predicted if the two drugs acted independently, in percentage points. Zero means the combination is exactly additive. As a working guide, scores above about 10 across the matrix indicate strong synergy, 3 to 10 moderate synergy, and below -3 antagonism — but the thresholds are conventions, not statistics, so judge them against your replicate variability.
How do you calculate a Bliss independence score?
Convert viability to fractional effect for each single agent, then the Bliss expectation is Ea + Eb − (Ea × Eb). The Bliss score for that well is the observed combination effect minus this expectation. A positive value means the pair did more than independent action predicts. This tool computes that for every well of the dose matrix.
Do I need a full dose matrix or is one combination enough?
One combination gives you one number with no way to tell signal from noise. A dose matrix lets you see whether synergy is consistent across the surface or confined to a single well, which is usually where an artefact hides. The single-agent row and column are required regardless, because the Bliss expectation is computed from them.
What is the difference between Bliss independence and Loewe additivity?
Bliss independence assumes the two drugs act through independent mechanisms and multiplies their probabilities of effect. Loewe additivity assumes they act through the same mechanism and asks whether the combination behaves like a dose of one drug diluted into the other. Bliss is the usual choice when mechanisms differ; Loewe is more appropriate for drugs hitting the same target, and the two can disagree on the same dataset. This tool computes Bliss only.
What counts as real drug synergy?
A positive Bliss score means the combination exceeded what independent action predicts, but a small positive value can arise from assay noise alone. Convincing synergy shows a consistent positive region across neighbouring dose pairs rather than isolated wells, is reproducible across independent experiments, and is large relative to the variability of your single-agent controls.