The invention relates to a system for injecting a liquid into a body, comprising an implantable chamber disposed under the skin and an injection device including: a base bearing an injection needle having a free end that is intended to be inserted into the chamber; an injection conduit connected to the needle; means for extracting the needle; means that can be actuated to generate excess pressure in the injection conduit; synchronization means between the extraction means and the generation means for actuating the generation means during a needle extraction phase. The synchronization means can essentially only actuate the generation means when the free end of the needle is located in the chamber during the needle extraction phase.