Sentence:  No sooner has a [synthetic] drug been blacklisted than chemists adjust their recipe and start churning out a subtly different one. These "legal highs" are sold for the few months it takes the authorities to identify and ban them, and then the <WSD>cycle</WSD> begins again. 
Meaning of the word marked with <WSD> tags:  A process that returns to its beginning and then repeats itself in the same sequence.