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Most antibiotics are bacteriostatic, meaning they inhibit the replication of bacteria. To achieve this effect you have to maintain a sufficient concentration of antibiotics over time. Taking double the recommended dosage would put you way over that concentration, but it wouldn’t achieve a greater inhibition on bacterial replication. As your body breaks down the substance, dosing with half the recommended frequency would put you below the minimal inhibitory concentration before your next dose, allowing some regrowth of the bacterial colony every day.

There are many factors that go into choosing dosing intervals. Some drugs are described as "bactericidal" vs "bacteriostatic". Some bactericidal drugs have a "concentration-dependent" response rather than a "time-dependent" response, which means that they are more effective when given a large dose for a short duration rather than a smaller dose for a longer time. Some drugs also continue to inhibit bacterial growth even when it is no longer detectable in blood (postantibiotic effect)

There is a class of drugs called aminoglycosides (tobramycin, streptomycin, amikacin) that are often taken at long intervals. They have an initial concentration-dependent bactericidal effect when first taken, then continue to provide bacteriostatic effect for many hours. Infrequent dosing also helps reduce toxic side-effects (hearing damage).

Also drug makers try to make drugs which can be taken as infrequently as possible while still being effective in order to improve patient compliance. They want you to take the drug as directed and not create a resistant super-bug. If the drug is effective when only taken once or twice per day, particularly in oral medication for an outpatient, this is preferable to a drug that is taken multiple times per day. Its just human nature to forget a dose here or there and to be less motivated to remember doses on time once you start feeling better.

Answering as a microbiologist, not a pharmacist or MD.

Different bacteria are able to survive different concentrations of antibiotics before they are killed. We call the lowest concentration that is enough to kill them the Minimum Inhibitory Concentration (MIC). In order to treat a patient, we want to make sure they have at least the MIC of the antibiotic, otherwise the bacteria will survive. Additionally, an antibiotic concentration below the MIC won't kill bacteria, but will still provide a stress and create a selective pressure for resistance. Then, upping the concentration to the original MIC won't matter - the bacteria will already be resistant to it, and the new MIC will be higher.

Now the other thing to consider is that once an antibiotic enters the body, it is gradually broken down. This means that if a patient is given a dose of exactly the MIC, the concentration will quickly drop to the point where it won't be helpful, and thus won't be cured. The idea to counter this is to give a higher dose than is needed, and to take a new dose around when the concentration would drop to the MIC. That way, there is always a lethal concentration of antibiotics in the patient's body until all of the disease-causing bacteria are dead (or at least, enough dead that the immune system can take care of any stragglers).

The last consideration is that most antibiotics can have a toxic effect on humans at a high enough concentration. So we want to plan a dose that is high enough to always be above the MIC, but low enough that it doesn't actually harm the patient.

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