The British company took the highly unusual step of suing the US Food and Drug Administration over its decision to allow Gensia Sicor, a small American firm, to produce a cheaper version of its anaesthetic Diprivan.
In a lawsuit filed in the US state of Maryland, Zeneca alleged that the FDA breached an exclusivity agreement and demanded a withdrawal of Gensia's licence.
The little-known US company last month won approval to produce a medicine which uses Diprivan's key ingredient in combination with other substances not contained in the Zeneca product.
The Gensia drug is to be launched later this year and is expected to cost less than Diprivan, an injectable anaesthetic sold to hospitals.
Industry experts believe that it could become a powerful competitor to Diprivan, which last year had sales of $300m in the US and $600m worldwide.
However, the UK group, which is completing its multibillion pound merger with Sweden's Astra, is claiming that the Gensia product breaches a previous agreement with the FDA.
According to Zeneca, the deal gives it exclusive rights over the sale of Diprivan and similar products until June of this year.
The British company is also alleging that Gensia's formulation of the drug could be harmful for patients.
Gensia said it had applied to intervene in the lawsuit between Zeneca and the FDA and added that it would "vigorously protect" its interests.