A BAN on synthetic cannabis has failed to stop its sale and may have caused stronger forms of the party drug to hit the market.

Sold as Kronic, Northern Lights, Spice and Amsterdam High, the herbal-chemical blend was in effect outlawed in July when eight cannabis-like chemicals were banned by Australia's federal medicines regulator.

Police struggle to control the fake weed market. Credit:Ken Irwin

But it is still being sold widely in Melbourne in ''legal high'' stores, tobacconists and sex shops, as suppliers have got around the law by reformulating brands with chemicals that have a similar effect but are not on the banned list.

Victoria Police told The Sunday Age the drugs were virtually impossible to control as there was no way to know what chemicals were in them without forensic tests.