Home Office minister Jeremy Browne said the worst cannabis is now about six or seven times stronger than it was a generation ago.
He said: "It's like somebody going from drinking a pint of beer to drinking a pint of neat vodka."
He added that despite falls in heroin use, he was concerned about new threats.
Mr Browne said: "Some of these legal highs can be very dangerous for people's health and of course the situations is mutating the whole time. All of these chemical compounds, these synthetic drugs, are being manipulated."
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Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.
Police chief says the video can now be release without 'adversely impacting' the investigation.