Zyprexa, or olanzapine, is a powerful drug used to treat schizophrenia and bipolar disorder, or manic depression. It is the world’s fifth-bestselling drug and generated sales of $4.2 billion (£2.1 billion) last year — about 30 per cent of the company’s total revenues.

Eli Lilly, one of America’s leading drugmakers, has been fighting a string of court actions related to the drug, which has been taken by 20 million people worldwide since its launch in 1996, including thousands in the UK.

A series of leaked court documents seen by The Times suggested that the company was aware of the risks of weight gain, hyperglycemia (high blood sugar) and diabetes associated with the drug but presented them as common problems across the whole class of schizophrenia drugs and not restricted to Zyprexa.

In one document dated October 9, 2000, Robert Baker, a senior Lilly clinical research physician, e-mailed colleagues about a meeting of an academic advisory board he had attended in Atlanta. It had “reinforced my impression that hyperglycemia remains quite a threat for olanzapine and may merit increasing even further medical attention and marketing focus on this topic”. Dr Baker added: “[The board was] quite impressed by the magnitude of weight gain on olanzapine and implications for glucose.”

Another internal document dated October 14-15, 1998, described the risk of weight gain as a “top threat” to Zyprexa.

This month Lilly agreed to pay up to $500 million to settle 18,000 lawsuits from people who claimed that they had developed diabetes or other diseases after taking Zyprexa.

Lilly has now agreed to pay out more than $1.2 billion to 28,500 people, who had claimed that their health was damaged by the drug. Another 1,200 lawsuits are continuing.

A statement from Lilly said that the documents represented “a tiny fraction of the more than 11 million pages of documents provided by Lilly as part of the litigation process. They do not accurately portray Lilly’s conduct nor represent an accurate view of Lilly company strategy or activities.”

It added that the documents were “selectively and illegally leaked”. The company had done more than 23 years of research on Zyprexa and had found no evidence of a causal link with diabetes. “The link between schizophrenia and diabetes pre-dates the development of any antipsychotic,” it said.

Eli Lilly added that Zyprexa’s labelling had always flagged concerns about weight gain and diabetes to physicians.

Valuable market

Schizophrenia drugs

Global market worth about $11 billion in 2004.

10 per cent annual growth

Leading products:

Eli Lilly: Zyprexa or Olanzapine; 2004 sales of $4.4 billion.

Johnson & Johnson: Risperdal or risperidone; 2004 sales of $3.1 billion

AstraZeneca: Seroquel or quetiapine; 2004 sales of $2 billion