Warrants were executed at a total of nine addresses by officers in Wigan and Salford on May 17
Two sophisticated labs, believed to be making phoney Diazepam pills, were discovered.
It’s estimated that both labs, on Lower Green Lane in Astley and Albion Street in Salford, may have been churning out thousands of tablets an hour.
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The operation, led by Greater Manchester Police’s Serious and Organised Crime Group (SOCG), was part of GMP’s commitment to taking drugs off the streets under Operation Cranium, a multi-agency response to prescription drugs and the sale of counterfeit medication.
Four men, aged 42 to 77, were all arrested on suspicion of conspiracy to supply class C drugs.
A 42-year-old was also further arrested on suspicion of conspiracy to possess a firearm with intent to endanger life.
Around £20,000 in cash and assets including Rolex and Omega watches worth around £30,000 were seized, along with three firearms and ammunition taken from the address in Astley are awaiting further testing to see if they are viable.
Det Chief Insp Jim Faulkner, from GMP’s SOCG, said: “Misuse of prescription drugs can lead to fatal consequences and there have already been several cases where people have become seriously ill or died as a result of acquiring them illegally.
“Illegal drugs and their distribution can not only blight communities, but can also pose a real harm and risk to both those in the community and those using and taking them.
"We work with partners including Public Health England and the Greater Manchester Drugs Early Warning System which monitors new and emerging drugs that pose a real threat.
“We will not hesitate in taking action on those involved and this is a strong example, whereby we’ve wiped out a clearly sophisticated set up and hopefully saved lives as a result.
"Today’s warrants are another huge step in our crackdown and it doesn’t stop here.”
Parts and mechanical items from inside the suspected labs are currently in the process of being dismantled and investigated.
DCI Faulkner added: “I can’t emphasise enough the dangers of taking these drugs without a relevant prescription and dosage guidance from a healthcare professional.
"Illicit supplies of prescription drugs may be counterfeit or adulterated and anyone purchasing them cannot be sure of their origin or what they may have been mixed with. Packages may also contain incorrect dosage information.
“Our best fight against those who would seek to profit from drugs supply are members of the public, on whom we rely on to report suspicious activity.
Bev Hughes, Deputy Mayor of Greater Manchester, said: “People’s lives can be destroyed by the misuse of prescription drugs and anyone taking advantage of this for their own gain needs to be found and brought to justice.
“Today is a really positive step forward to not only take action against those responsible for these crimes, but to also help get control of an illegal drug market that has such a negative impact on our communities.
“Police will continue to gather intelligence and crackdown on markets like this which lead to the deterioration of neighbourhoods and increased anti-social behaviour and violence.”