synergism

in toxicology
https://doi.org/10.1351/goldbook.S06229
Pharmacological or toxicological interaction in which the combined biological effect of two or more substances is greater than expected on the basis of the simple summation of the toxicity of each of the individual substances.
Source:
PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2103 (https://doi.org/10.1351/pac199365092003)