。
Drug addiction is a cancer on modern society.
Literal
Drug-addiction [topic-は] modern-society [possessive-の] cancer.
癌 (がん, 'cancer') is used metaphorically — a societal disease eating away at the fabric of civilization. 麻薬中毒 ('drug addiction') is a formal compound: 麻薬 ('narcotics') + 中毒 ('addiction/poisoning'). The sentence has an essay-like or editorial tone.