An adapter is provided with a central, longitudinal, inner through hole and has a rounded off tip-part to be inserted into the external urethral orifice, a sealing collar and a cylindrical connecting part known connectable to the appropriate part of a dosing device, advantageously to the ISO 594 standard Luer Slip or Luer-Lock tip of a syringe. The adapter is connected by known method to the appropriate part of a dosing device containing the active and/or contrast agent, then the active and/or contrast agent is ingested leakage-free into the urethra or into the urethra and the bladder through the adapter, then through the external urethral orifice by using overpressure in the dosing device and inserting the tip-part into the external urethral orifice and squeezing the sealing collar in a proper way to the external urethral orifice.