As a child you may remember being told by your doctor to finish your course of antibiotics so that the bug does not become resistant…. I think such a myth still exists today, possibly to a somewhat lesser extent. In reality, a longer than necessary course of antibiotics just prolongs the selection pressure for resistant organisms to thrive and thus leads to antibiotic resistance.
In fit healthy people, antibiotic courses for straightforward UTIs, URTIs, simple soft tissue infections etc., can often be remarkably short (if antibiotics are needed at all), and people may notice a significant improvement in their symptoms after just one or two doses.
It is also important to note that the patient’s immune system usually works in tandem with antibiotic therapy so when the pathogenic bacterial load has been reduced by the first few doses of antibiotics it makes it easier for a patient’s immune system to complete the job of controlling the infection.
The real message to get across here is that antibiotic courses should be of sufficient duration to deal with the infection but not so prolonged that it unnecessarily promotes selection of resistant bacterial strains.
For most antibiotic courses, the maximal effect usually occurs after the first couple of doses and then any further effect tails off very quickly after this. I occasionally come across patients both in the hospital and community setting where the patient has been on the same antibiotic for weeks if not months to treat a particular infection. Only very rarely is this indicated, and in my opinion should never be done without consultation with a specialist.