Every time a doctor gives you antibiotics, they ask you to finish all of them, even if you might feel fine after taking only part of the package.

Most searches on Google says that if I don't finish them, the bacteria will become immune to the antibiotic. Is this true?

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    @Sklivvz Where in that question does it state this? The only reference I can find to antibiotics is in the question itself. The premise of this question is sound but it needs to cite an example of the claim. Commented Sep 28, 2011 at 13:20
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    Sklivvz, you haven't shown the logic of that closure. And my doctor friends tell me very different reasons for needing to finish courses of antibiotics. Commented Sep 28, 2011 at 13:50
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    Gee guys... google.co.uk/…
    – Sklivvz
    Commented Sep 28, 2011 at 20:22
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    How do you answer a question that says "all of the available research says X; it X true?" It's not that bacteria "become" resistant, it's that some small proportion of the bacteria are resistant (genetic variation simply is) and if you cease treatment before they're killed off (they're resistant, not immune), the entire regrowth of the population will descend from the resistant bugs. That makes treatment of the recurrence (or spread) more difficult -- and some proportion of the new population will be more resistant than the parent population, [GOTO 10]... Commented Sep 28, 2011 at 21:16
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    The question appears to conflate two separate issues: overuse of antibiotics and whether you should complete a course once prescribed them. There are good reasons for overuse (by which I mean prescribing when it isn't necessary) to drive resistance, but the reasons you should complete a course are unrelated and more to do with making sure the infection cannot recur.
    – matt_black
    Commented Sep 28, 2011 at 23:23

1 Answer 1


Doctors do always tell you to finish your course of antibiotics, even if you feel better. The reason for this is that you feel better before all the bacteria are killed within you body. If you stop the treatment early, the bacteria will start to grow again and the infection will recur. It is also likely that the second attack will consist of bacteria with more immunity to the antibiotic (the ones without any resistance were killed, right?) and so will be worse. However there are other reasons apart from the immunity issue. Here is a good article on the subject. And another. And another. And another.

Other factors also contribute to the rise of antibiotic-immune bacteria, including the overuse of antibiotics for minor illnesses.

You edited to ask if bacteria "become immune" to the antibiotics. Its not clear what you mean by "become immune" but the answer is still probably "yes". The second wave attack by resistant bacteria is due to two effect. Selection is the primary one. Some of the bacteria in the infection are more resistant to your prescribed antibiotic than others. A partial course removes the bacteria that have low resistance and leaves those with high resistance, which form the 'second wave' infection that results from stopping the course early. In that case those bacteria are not "becoming resistant", it's just that the resistant ones predominate. However it is also true that mutations can and do occur, and this can result in bacteria that previously had low resistance becoming highly resistant. In that sense bacteria sometimes are "becoming resistant". This paper gives more information.

  • Not only “more immune” but potentially just “immune” so that the antibiotic doesn’t help any more at all and you need to switch to a different one. Commented Sep 29, 2011 at 15:29
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    I said "more immune" deliberately, because immunity isn't an on/off thing. An antibiotic can be more effective or less effective against a strain of bacteria. Commented Sep 29, 2011 at 15:56
  • Hmm, that’s true. Commented Sep 29, 2011 at 15:57
  • Good answer, but could be still improved by blockquoting linked articles to make verification easier.
    – Suma
    Commented Sep 29, 2011 at 22:23

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