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 Sisyphean task looks easy next to the fight against <WSD>synthetic</WSD> 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:  produced by synthesis instead of being isolated from a natural source (but may be identical to a product so obtained)