Piston syringe, in particular a disposable piston syringe, wherein the piston syringe has - a syringe body, which has a syringe barrel for receiving a preferably oxybutynin-containing composition, and an adapter for connection to an instillation device, wherein the syringe body is formed in one piece and/or wherein the adapter is firmly and/or non-detachably connected to the syringe barrel, and - a syringe plunger with a plunger stopper; wherein the adapter has at least one first section (A 1) with an outer circumference and/or outer diameter tapering continuously in the outlet direction, and wherein the adapter has at least one second, cylindrical section (A 2) directly adjoining the first section (A 1) in the outlet direction with a reduced outer circumference compared to the first section (A 1), wherein the second section (A 2) extends towards an outlet opening of the adapter and the outlet opening is formed at the free end of the second section (A 2), wherein the first section (A 1) tapers from a maximum outside diameter (D max1) of the first section (A 1) in the outlet direction to a minimum outside diameter (D min1) of the first section (A 1) and the ratio of the maximum outside diameter (D max1) of the first section (A 1) to the minimum outside diameter (D min1) of the first section (A 1) is in a range from 1,1 to 1.6, characterized in that exactly one step (offset, stage) is formed at the transition from the first section (A 1) to the second section (A 2), wherein at least one third section (A 3) of the adapter is provided directly upstream of the first section (A 1), wherein the third section (A 3) has an outer circumference and/or outer diameter that tapers continuously in the outlet direction, wherein the third section (A 3) merges smoothly into the first section (A 1) and wherein the ratio of a maximum outer diameter (D max3) of the third section (A 3) to a minimum outer diameter (D min3) of the third section (A 3) is in a range from 1.01 to 1.06. wherein the piston syringe has - a syringe body, which has a syringe barrel for receiving a preferably oxybutynin-containing composition, and an adapter for connection to an instillation device, wherein the syringe body is formed in one piece and/or wherein the adapter is firmly and/or non-detachably connected to the syringe barrel, and - a syringe plunger with a plunger stopper; - a syringe body, which has a syringe barrel for receiving a preferably oxybutynin-containing composition, and an adapter for connection to an instillation device, wherein the syringe body is formed in one piece and/or wherein the adapter is firmly and/or non-detachably connected to the syringe barrel, and - a syringe plunger with a plunger stopper; wherein the adapter has at least one first section (A 1) with an outer circumference and/or outer diameter tapering continuously in the outlet direction, and wherein the adapter has at least one second, cylindrical section (A 2) directly adjoining the first section (A 1) in the outlet direction with a reduced outer circumference compared to the first section (A 1), wherein the second section (A 2) extends towards an outlet opening of the adapter and the outlet opening is formed at the free end of the second section (A 2), wherein the first section (A 1) tapers from a maximum outside diameter (D max1) of the first section (A 1) in the outlet direction to a minimum outside diameter (D min1) of the first section (A 1) and the ratio of the maximum outside diameter (D max1) of the first section (A 1) to the minimum outside diameter (D min1) of the first section (A 1) is in a range from 1,1 to 1.6, characterized in that exactly one step (offset, stage) is formed at the transition from the first section (A 1) to the second section (A 2), wherein at least one third section (A 3) of the adapter is provided directly upstream of the first section (A 1), wherein the third section (A 3) has an outer circumference and/or outer diameter that tapers continuously in the outlet direction, wherein the third section (A 3) merges smoothly into the first section (A 1) and wherein the ratio of a maximum outer diameter (D max3) of the third section (A 3) to a minimum outer diameter (D min3) of the third section (A 3) is in a range from 1.01 to 1.06.