First, the headline is that synthetic cannabis use is declining. That’s good, but it’s in a survey of frequent users of illegal drugs. If you have the contacts and willingness to buy illegal drugs, it isn’t surprising that you’d prefer real cannabis to the synthetics — there seems to be pretty universal agreement that the synthetics are less pleasant and more dangerous. This survey won’t pick up trends in more widespread casual use, or in use by teenagers, which are probably more important.
Second, the study describes the problems caused by much more toxic new substitutes for Ecstacy and LSD. This is one of the arguments for legalisation. On the other hand, they are also finding increased abuse of prescription oxycodone. This phenomenon, much more severe in the US, weakens the legalisation argument somewhat. Many people (including me) used to believe, based on reasonable evidence, that a substantial fraction of the adverse health impact of opioid addiction was due to the low and unpredictably-varying purity of street drugs, and that pure, standardised drugs would reduce overdoses. As Keith Humphreys describes, this turns out not to be the case.