You’ve been given an antibiotic by your GP for a respiratory infection. The course goes for 4 days and you still feel unwell ! You return to your medico and he/she gives you another course.
This really makes little sense. In the micro lab , we perform susceptibility tests for bacteria to find out whether they are susceptible to common antibiotics. In essence, your GP has just performed an in vivo susceptibility test that has failed the test. The cough or cold or sore throat illness has not responded to antibiotics. This means most probably that a virus has been responsible and viruses do not respond to antibiotics as all will know. Hence repeating the course will achieve little and may expose you to risk of side effects or secondary infection with something worse.
When the in vivo test fails, possible alternatives to a viral (non-bacterial) cause include a condition that is not due to an infection (eg. an allergic condition) or, unusually for a respiratory infection, an infection caused by a bacterium that is resistant to the antibiotic you have been prescribed.
Even when the in vivo test works (you respond to the antibiotic), remember that many infections are self-limited – that is, they get better whatever you do. And so, it may be the case that the antibiotic had nothing to do with you getting better and your body’s immune system was up to it!