Aventis is considering using poison pills as part of its defence against the hostile takeover bid from Sanofi-Synthélabo, a rival French pharmaceuticals group.

Referring to undisclosed relations with other groups, which could lapse if the company's ownership changes, Igor Landau, chief executive of Aventis, said: "These agreements are confidential, but important enough to be taken into consideration by our shareholders. We will quantify them when the right moment comes."