If you had a cold and was proscribed amoxycillin but all ready have a box of flucloxacillin could you just use that?

Cold: You probably do not need an antibiotic to begin with, but if you really do, flucloxacillin is meant only for Staphylococcal infection, while amoxicillin is a more wide spectrum compound meant for many of the organisms present in the upper respiratory tract. Neither is likely to help your cold.

Antibiotic choice: It might not make much difference to you, but your doctor should be aware of your decision. THis is especially true if you develop a reaction to, or an allergy from the "substitute" medication. While rare, if they happen they need to be reported.