A group of bereaved parents in America have joined together to sue GlaxoSmithKline (GSK), the UK's largest drug company, for selling Seroxat as safe and effective in treating depressed children even though clinical trials had produced worrying results. The company's statements to the public did not contain the whole truth. As one uncovered e-mail said: "Essentially the study did not really show [Seroxat] was effective in treating adolescent depression, which is not something we want to publicize."

Whatever marketing ethic guided GSK and its advisors, the requirements of evidence in a law court are more rigorous. The form of oath now taken in law courts was approved by the King's Bench judges in January 1927 for use in the civil and criminal courts. It is that: “that the evidence I shall give shall be the truth, the whole truth, and nothing but the truth”. That formula is now required under s 1(1) of the Oaths Act 1978. The phrase "the truth, the whole truth, and nothing but the truth" is used extensively in other jurisdictions and is arguably the most commonly incanted piece of legal jargon in the world.

The phrase owes much to the Anglo-Saxon penchant for the rhythmical power of language - the sort used in the tenth-century poem Beowulf. It appears to have been used irregularly in many English regions since those times but the chant was first given official recognition in The Book of Oaths 1649, a time when the role of the witness was becoming increasingly important.

The words "the whole truth" were inserted in order to deny an assertion of the 13th-century theologian Thomas Aquinas that an oath to tell the truth did not oblige a person to tell the whole truth.

In law, though, it is important for those testifying to tell the whole truth. It must not be misleadingly cut or pared down. Imagine a person says he went to a gang fight taking with him a stick, and that this is true. His statement would have limited value if he omitted to tell those concerned with the gruesome aftermath of a terrible fight that he had also taken a machete and a gun.

In 1992, three former executives of the firm Matrix Churchill were prosecuted at the Old Bailey for exporting equipment with a munitions potential to Saddam Hussein’s dictatorship in Iraq. A question arose as to why the minister for trade, Alan Clark, had approved licences for such exports. In court, he admitted having been economic with the truth in omitting reference to British machines being used to make munitions in Iraq when he discussed export licences with the manufacturers.

He said that it was neither dishonest nor misleading, although he knew that some machine tools were destined for President Saddam Hussein's munitions factories. A minute of the meeting records Mr Clark describing the machines as being for ''general engineering''.

He was asked by Geoffrey Robertson, QC, if he was being economical with the truth. Mr Clark replied: ''With the actualité.'' Then he added: ''All I did not say was, 'and for making munitions', if I thought they were.'' A minister, however, who approved the sale of equipment to the vicious Saddam Hussein for “general engineering” knowing that it would probably be used as weaponry would be described by many people in a couple of words neither of which is French.

Truth is a precious substance that can be given generously and used by everyone without it ever running out. Certainly, there is no need for it to be used sparingly in any legal setting.

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