An apparatus (200) attachable to a syringe or pen injector (100) for detecting and logging a dose which has been set and/or injected. The apparatus determines dose numbers from the dose scale of the syringe or injector e.g. optically (e.g. via OCR, RFID, pattern recognition) and comprises a control unit (210) configured to analyze the extracted injection amount absolute value and a storage unit (220) configured to store the information.