An explosion of hundreds of new “legal highs” in recent years has left governments around the world in the dust as lawmakers struggle to keep prohibition laws updated as more and more never-before-seen drugs flood the black market.

The United Nations Office on Drugs and Crime (UNODC) said Wednesday that more than 251 new psychoactive substances were available on the black market by mid-2012, a growth of more than 50 percent over 2009. The UNODC’s 2013 World Drug Report finds that these substances are “proliferating at an unprecedented rate and posing unforeseen public health consequences.”

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The situation has become so severe that the study says the number of new psychoactive substances available on the black market today exceeds the number of controlled psychoactive substances currently prohibited by governments around the world.

One of the problems governments are having with these rapidly emerging substances is that when they place one drug on a schedule of controlled substances, chemists slightly tweak the molecular structure and re-release the drug, once again technically legal thanks to tiny changes.

“There is a lack of long-term data which would provide a much-needed perspective: no sooner is one substance scheduled, than another one replaces it, thus making it difficult to study the long-term impact of a substance on usage and its health effects,” the study notes.

In other words, prohibition laws have now created a game of cat-and-mouse between authorities and the developers of untested new drugs, and the drug developers are winning. A similar effect was observed in the U.S. after authorities banned the psychoactive chemicals in the marijuana-substitute “Spice,” leading to hundreds of variants of the synthetic drug, some with terrifying physical and psychological side-effects.

The UNODC study said that the global body is now coordinating a rapid-response system that will warn governments of these new drugs as they are discovered on streets around the world.

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“We have agreed on a path for our ongoing discussion,” UNODC’s executive director, Yury Fedotov, said in an advisory. “I hope it will lead to an affirmation of the importance of the international drug control conventions, as well as an acknowledgement that the conventions are humane, human-rights centered and flexible. There must also be a firm emphasis on health and we must support and promote alternative sustainable livelihoods. It is also essential that we recognize the important role played by criminal justice systems in countering the world drug problem and the need for enhanced work against precursor chemicals.”