About an antibiotic　抗生物質について

Injection or intravenous antibiotics may be used in situation where oral antibiotics have failed.
Why is oral antibiotic alone not always effective?

In order to kill all of the bacteria, adequate levels and certain time of the antibiotic in the blood should be achieved and kept.

Oral antibiotics need to be absorbed by intestines and then enter the bloodstream. Therefore, the levels of the antibiotics in the blood can be affected by many factors. Unlike oral antibiotics, intravenous injection with the same dose can achieve the adequate level and keep certain time in the blood because the absorbing procedure is omitted and the dose can directly enter the bloodstream.

Thus, injection or intravenous antibiotics may be used in situation where oral antibiotics have failed.