A brush, in particular a toothbrush, comprising a head part with a cleaning structure featuring bristles, a hand part and a neck part, and a cavity located in the head or neck part comprising at least one outlet opening for an active substance. In one embodiment, the cavity incorporates an active substance element, which contains the active substance in a carrier material and releases said substance in a controlled manner when it comes into contact with water. In another embodiment, the cavity is adapted to the shape of an active substance element by means of defined external contours. The volume of the cavity is preferably 1-2 times and no more than 4 times greater than the volume of the active substance element, ensuring that the latter is preferably surrounded by water during use. This allows the active substance to be effectively metered and ensures the brush is easy to use.