An MSP today called for answers after it emerged that a man and woman accused of exporting huge amounts of a date rape drug to the US have escaped prosecution.

Denise Macpherson, 47, and Hanan Rabin, 55, were originally charged after £4.4 million of the drug GBL - gamma-butyro lactone - was seized in West Lothian.

GBL is used to make the date rape drug GHB - Gamma-hydroxybutyrate - which is illegal in the UK and the US.

While GBL is outlawed in the US it is legal in the UK.

Macpherson and Rabin were charged with exporting GBL to the US between April 19 and May 24 2006.

They were also accused of supplying the substance from Macpherson's home between the same dates.

But the Crown Office said that after "full and careful consideration of all the evidence available" prosecutors decided to drop the proceedings in October 2007.

Labour's justice spokesman Richard Baker today called on Scottish Ministers to explain why the case has been dropped.

He said: "People know the horror that rape causes.

"Everything must be done to make sure those who profit from the supply of date rape drugs are dealt with.

"I urge Scottish Ministers to examine what has gone wrong in this case and see what action now needs to be taken to be sure that such cases can be effectively prosecuted in the future."

Macpherson and Rabin were arrested following raids led by the Scottish Crime and Drug Enforcement Agency (SCDEA) on Macpherson's home and at the Eucal Business Centre on Craigshill Road in Livingston.

GBL is used to make GHB and is often sold by itself. It is orally ingested and turns into GHB in the body, having the same effect.

Also known as liquid ecstasy, GHB is an odourless, colourless liquid sedative which became popular in dance culture.

The Home Office made GHB illegal in 2003, reclassifying it to Class C status.

It is a powerful anaesthetic which effectively "knocks out" victims and leaves them with no recollection of what has happened.

Because the drug, which can be made in home-based labs using solvents and caustic soda, is difficult to detect in the blood, prosecutions are difficult.

Clubbers also use the substance to create a feeling of euphoria as in small quantities it acts as an anti-depressant.