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 "legal high" in April, making it a Class B drug.
Former chief drugs adviser Prof David Nutt said the test results undermined the reasons behind the ban.
But Professor Les Iverson, the current chairman of the Advisory Council on the Misuse of Drugs (ACMD), said the decision to recommend a ban on mephedrone was based on "thorough research".
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".
At the time, police believed the pair, both from Scunthorpe, 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.
At the time, the ban on mephedrone - which catagorised it alongside amphetamines and cannabis - caused a row amongst some politicians and scientists.
The Advisory Council for the Misuse of Drugs (ACMD) had recommended a ban, saying the substance was "likely to be harmful" despite incomplete research.
But the leading medical journal The Lancet questioned the ban, saying it had been rushed and politics had been allowed to "contaminate" science.
Two members of the committee quit in quick succession during the row.
One of them, Eric Carlin, told BBC Radio 4's PM programme the decision to ban the drug should be "revisited" in light of the findings, and the "public health consequences" of the ban needed to be considered.
"The fact these two people died and it's not actually connected with mephedrone just emphasises the fact that we were under a lot of pressure to ban this drug and these cases were actually cited as being examples of why that was necessary," he said.
Prof Nutt, who was sacked by the then Home Secretary Alan Johnson in October 2009, said the findings were "embarrassing" for the government, media and police.
"If these reports are true, the previous government's rush to ban mephedrone never had any serious scientific credibility - it looks much more like a decision based on a short-term electoral calculation.
"This news demonstrates why it's so important to base drug classification on the evidence, not fear, and why the police, media and politicians should only make public prouncements once the facts are clear."
Campaign group the UK Drug Policy Commission said it hoped there would be a "fresh look at different ways of formulating drug policy".
Chief executive Roger Howard said: "We believe the new government has an opportunity to both better protect the public and save money by reshaping the architecture for decision-making.
"Getting the governance right will lead to better outcomes for every community in Britain, and rebuild trust between experts and politicians."
However, ACMD chief Prof Iverson said: "The ACMD gathered evidence from a number of experts and thoroughly researched the cathinones including mephedrone before making its recommendation.
"On the basis of this evidence, and in comparison with similar substances, it concluded that the harms associated with cathinones equate to other dangerous substances in Class B, particularly amphetamines which are structurally similar and act on the central nervous system in the same way."
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.
On Thursday, a coroner in Brighton said Mr Stirling Smith, who was 46 and had underlying health problems, died after after "injecting mephedrone repeatedly".
The European Union has also ordered a report into the health and social risks of mephedrone from the European Monitoring Centre for Drugs and Drug Addiction.