Abstract: A plunger for a syringe may include a plunger head for fluid dense limiting a fluid chamber of the syringe, and a piston rod fixed to the plunger head. The piston rod may be formed by longitudinally extending axial webs that have an outwardly radial extension from the longitudinal axis and lead into a common crossing section. A free activation edge, which radially protrudes the radial extension may be provided on the piston rod and engageable with a vibration activator of a hollow body of the syringe to emit an acoustic signal. A swinging arm body may be arranged between two axial webs and couple the activation edge with the crossing section such that in case of a vibration activation in an axial direction the swinging arm body has a radial swinging arm length between the activation edge and the crossing section of at least 40% of the radial extension.