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 synthetic drugs. No sooner has a drug been blacklisted than <WSD>chemists</WSD> adjust their recipe and start churning out a subtly different one. 
Meaning of the word marked with <WSD> tags:  A person who specializes in the science of chemistry, especially at a professional level.