Toxicology tests have shown that two teenagers whose deaths were linked to mephedrone had not taken the drug.
The deaths of Louis Wainwright, 18, and Nicholas Smith, 19, in March 2010 sparked concern about the synthetic stimulant, which was then legal.
The Labour government banned the so-called "legal high" in April, making it a Class B drug alongside amphetamines and cannabis.
But tests have revealed there were no traces of mephedrone in their blood.
It is thought further tests are being conducted to try to establish what, if any, substances the pair had taken.
Mephedrone - also known as Meow, Bubbles and M-CAT - is derived from cathinone, a compound found in a plant called Khat.
Humberside Police, which carried out the initial investigation into the teenagers' deaths, said in March it had "information to suggest these deaths are linked to M-CAT".
Its statement went on to say: "We would encourage anyone who may have taken the drug to attend a local hospital as a matter of urgency."
At the time, police believed the pair had been drinking and had also taken methadone - a similar-sounding but completely different drug to mephedrone.
On Friday, a spokeswoman said the force could not confirm or deny the results of the toxicology tests.
She said: "The pathology report, which includes toxicology findings, is prepared on behalf of the coroner and is not yet complete.
"The findings of the report, once completed, will be forwarded to the coroner and may be discussed at any inquest and will not be disclosed without the authority of HM Coroner."
North East Lincolnshire Coroners Court has refused to comment ahead of the inquest.
Mephedrone has been implicated in the deaths of 34 people in the UK - 26 in England and eight in Scotland.
But so far, the drug has been established as a cause of death in only one case in England, that of John Stirling Smith.
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