City scientist dismisses Maddie drugs claim
Unconfirmed reports have also alleged that tests on a liquid - possibly blood - found in the family's hire car suggest the young girl might have overdosed on sleeping tablets.
Toxicological analysis showed Madeleine consumed a "significant" quantity of the pills, according to unnamed sources in Portugal reported in various newspapers.
Samples of the missing youngster's hair were also found in the vehicle, one source has said previously.
However Allan Jamieson, of the Forensic Institute in Edinburgh, said it would be impossible to detect a one-off dose of sedatives in a hair sample as it would not have been in the body long enough.
Mr Jamieson, who has testified about the reliability of DNA at several high-profile cases, made the comments in an interview with Tonight with Trevor McDonald, to be broadcast on ITV1 at 8pm tonight.
"It's reasonably well-known you can use hair to look at the drug use of an individual over a period, but that really wouldn't extend to just a few minutes of use before the hair was sampled," he said.
Asked if there was any way of telling from a sample if a person had died from taking the drug, he said: "No I don't think so. The effect of the drug is much quicker than its metabolism into the hair."
And when asked if it is possible to tell whether a hair sample has come from a living person, he said: "I am not aware of any way that you could tell that, so, in my opinion, it would be nonsensical to say that you can tell that."
Mr Jamieson, who is also the chair of the Standards Committee and Education Group of the Forensic Science Society, also said it was not possible to detect to tell the concentration of a drug from a dried blood spot - such as those reportedly found in the hire car boot.
He said: "If you have a volume of blood, then you can calculate the concentration in the blood but if all you have is a dried blood spot, then you wouldn't know the volume of blood that created that, therefore you would not know the concentration of drug in the blood. That is important in terms of its toxicological effect."
Asked whether it was possible to tell whether blood has come from a living person or "leaked" from a corpse, Mr Jamieson said: "Well blood from a living person you would expect to be neat blood.
"When someone dies, the body begins to break down and it is possible that blood could become mixed with other body fluids.
"But how you would establish that it had become mixed with other body fluids, I don't know how you would do that."
Meanwhile, lawyers for Gerry and Kate McCann have been working to dismiss allegations that a police dog picked up the "scent of death" on Mrs McCann's clothes.
The legal team have consulted the lawyers of an American man accused of murdering his estranged wife in a case where cadaver sniffer dog evidence was key, a source said.
Two British sniffer dogs, one capable of detecting blood and human remains, were brought to Portugal in early August.