The invention relates to a pump (2) intended to pump an additive in an SCR system for a vehicle. The pump is configured to rotate in a first direction of rotation in order to convey additive stored in a tank towards an injector via an injection channel. The pump includes a chamber (23) which houses a gear system (22). The chamber (23) is in fluid communication with the tank and the injection channel via an inlet channel (24) and an outlet channel (25) respectively. The pump is such that the inlet channel and the outlet channel are arranged so that after draining the injection channel, the chamber collects and retains the additive.