After Louis Wainwright, 18, and Nicholas Smith, 19, from Scunthorpe, Lincolnshire, died in March there was a clamour to ban the so-called legal high. It was made illegal last month by the former government, becoming a class B drug, along with cannabis and amphetamines.
But according to the BBC, toxicology reports showed that neither of the teenagers had mephedrone in their blood when they died.
More tests were being carried out to establish what, if any substances, the pair had taken.
An inquest will be held later at North East Lincolnshire Coroners Court.
Humberside Police, which is investigating the deaths, originally said it had information to suggest they were linked to mephedrone, also known as M-Cat, miaow miaow and bubbles.
In a statement at the time, it advised anyone who had taken the drug to seek medical treatment urgently.
Following the BBC report, a police spokeswoman could not confirm or deny what the toxicology tests had found. 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."
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