In a lawsuit filed in a New York court, the British group alleged that the US drug giant "systematically and deliberately attempted to mislead doctors" on the benefits of Evista, an osteoporosis drug.
According to Zeneca, Lilly's sales representatives told physicians that the drug helped to reduce the risk of breast cancer, even though it had only been approved as a treatment for osteoporosis - a bone-thinning disease which affects middle-aged women.
The UK company is the maker of Nolvadex, the leading product for the treatment of breast cancer, and one of Zeneca's best selling drugs. Last year Nolvadex sales totalled $369m in the US and around $500m worldwide. The drug is used by around 400,000 US patients and last year received approval to be used to prevent the disease in high-risk women.
The lawsuit said that Lilly's false claims prompted a number of doctors to prescribe Evista for breast cancer, leading to a drop in the sales of Nolvadex.
According to a Zeneca survey of over 500 doctors, one in three physicians were led to believe that Evista could be used as an alternative to Nolvadex. .Zeneca, which is merging with its Swedish rival Astra, claimed that Lilly's campaign created "serious risk to public health" as more and more women are using Evista instead of Nolvadex. The British company is seeking undisclosed damages and wants Lilly to pay for an advertising campaign to deny the Evista cancer claims.
A spokesman for Lilly yesterday denied Zeneca's allegations: "We don't feel there is anything inappropriate in our promotion."