Last resort medicine. That patient?

If an OCD and anxiety psychiatric patient was given last resort medicine during treatment twenty years ago.  Now, twenty years later, if he became mentally ill again. wouldn't other medicines respond now due to last resort medicine? There is no old prescription either.  Will first liner, 2nd liner drugs not work to treat anxiety patient. I suspect that last resort medicine may have been fed to the patient and fearing that no other medicine respond now.  Please guide me. What to do if last resort medicine has been fed to the patient and medicine name is not known to me.what will psychitrist do now to treat that  patient? Is the patient is not curable because he was fed last resort medicine 20 years ago.He is now suffering from GAD. Please provide detail information .