co·car·cin·o·gen 
(kō′kär-sĭnə-jən, kō-kärsĭn-ə-jĕn′)
(kō′kär-sĭnə-jən, kō-kärsĭn-ə-jĕn′)n.
A substance or factor that will not promote cancer by itself but can potentiate cancer when acting with carcinogenic agents.
co·car′cin·o·genic (-sə-nə-jĕnĭk) adj.