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 microbiology 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 on or more of:
- a condition that is not due to an infection (eg. an allergic condition)
- unusually for a respiratory infection, an infection caused by a bacterium that is resistant to the antibiotic prescribed
- the patient did not take the prescribed treatment
- the antibiotic did not penetrate the site of infection in sufficient quantity (most frequently this is into an abscess (infection collection) or within the brain e.g. in meningitis)
- there was a failure of “source control” (e.g. drainage of an abscess or removal of an infected device/prosthesis). See also this discussion about “scalpelmycin“.
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!
AIMED - Let's talk about antibiotics 
[…] antimicrobial susceptibility is measured and the limitations of lab testing. Understand that the ‘in vivo’ susceptibility test does not always get it […]
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Hi my name is Anna Hagerman and I have a sinus infection and it’s a bacteria infection with green and yellow muses so I had this infection for going on three weeks now and have been treated with three rounds of antibiotics and nothing is working so I want to know what you think don’t you think by now they would have try the top antibiotics on me because the other have failed please let me know your feed back.
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This is appallingly biased and dangerous ‘advice’.
The most likely reason for antibiotics not working (these days) is that because of medically biased and incompetent advice like that set iut here – the antibiotic – (properly presribed for a bacterial – NOT – virul infection- isn’t as biologically effective as it once was.
If tbe 1 week course of eg Amoxylin 500mg has only been partially effective in clearing the bacterial infection – it means it is appropriate either to continue the course for a further one week to fully clear the bacterial infection – or – a doctor / dentist should consider prescribing a stronger antibiotic (eg Augmentin) to ensure that the bacterial infection is cleared before stopping the antibiotics.
Stopping the antibiotics when they have only partially worked – because of some DUMB idea that continuing to prescribe beyond 1 course – is exactly the same as the danger of not completing a course of antibiotics in the first instance.
That caused serious potential health issues for you as the patient – and is the prime reason for the increase in Anti Biotic resistance over the last decade in particular since this MORONIC pseudo-medical advice was published online.
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I have an infected tooth and need antibiotics and I want to know after I take them will the infection be gone or do I have to pull the tooth out right away or how long can I wait ? I took prolix one month ago
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