An interaction between two or more factors in which their combined effect on an outcome is greater than the effect expected from summing (or otherwise combining according to a prespecified scale) their individual effects; determination of synergy requires specification of the effect measure and comparison rule (additive, multiplicative, etc.).

Principle

Whether an interaction qualifies as synergistic depends on the chosen scale and null model; demonstrating synergy requires explicit definition of the expected combined effect and empirical comparison to that expectation.

Demonstration

Illustrative scenario — Situation: Two low‑dose toxins each cause a 5% risk increase alone. Recognition: On the additive scale the expected combined increase is 10%. Action: An experiment finds a 30% increase when both are present. Consequence: The observed supra‑additive effect meets the predeclared criterion for synergy and implies prioritizing combined exposure control.

Misapplication

Labeling any statistical interaction as synergy without specifying scale or expected baseline. The error is conflating statistical interaction, scale dependence, or nonlinearity with mechanistic synergy.

Consequence

Identifying true synergy changes risk assessment and prevention priorities (combined exposure mitigation may yield greater benefit than addressing factors individually); misidentification can misallocate resources or mislead hazard communication.

Reversal

An interaction that is synergistic on one scale (e.g., additive) may appear antagonistic or null on another (e.g., multiplicative); dose, timing or population differences can reverse apparent synergy.

Boundary

Clearly within: combined effect exceeds prespecified expected effect based on the chosen null model. Boundary case: combined effect equals expectation (no interaction). Clearly outside: independent effects where combined outcome matches the null model.

Semantic Tension

Public‑health emphasis on additive excess versus mechanistic emphasis on multiplicative biological interactions; choosing a scale reflects different practical and theoretical priorities.

Synthesis

Synergy is an operational claim about deviation from a stated expectation; it requires explicit scale, predeclared null, and attention to dose, timing and population context.