Sentence:  As the world's drug habit shows, governments are failing in their quest to monitor every London window-box and Andean hillside for banned plants. But even that <WSD>Sisyphean</WSD> task looks easy next to the fight against synthetic drugs. No sooner has a drug been blacklisted than chemists adjust their recipe and start churning out a subtly different one. 
Meaning of the word marked with <WSD> tags:  Incessant or incessantly recurring, but futile.