If the patient has a particular preference for a specific type of local anaesthetic, view / type it in the Ana. Pref text box at bottom right of the Medical tab screen:

All changes are audited

Each time that you change this field, the change is written to the medical history list in the form of Date/time, User, and a comment "Ana. Pref changed to xxxx"

The Entry is automatically marked as void, so it only shows when you tick the Show Audit check box.