Why are antibiotics given every 8 hours?

Drug dosage interval is determined by how quickly it is removed from the blood by the kidneys or liver. This can vary with different antibiotics from every few hours to much longer. An every eight hours dosing would be required if that interval is what is required to keep a high enough blood level of the med to kill the germs.

Not all antibiotics are taken every 8 hours. It varies between antibiotics. Sometimes every 4-6 hours; others twice a day; and some once daily or less frequently. It depends on the chemical differences between antibiotics and sometimes on the particular infection being treated. Also, precise timing often isn't important. Every 8 hr usually can be 6-10 hr, as long as 3 doses are taken every 24 hr.

To maintain a certain level of antibiotic in your bloodstream. Antibiotics interfere with bacterial replication, and since they all don't divide at the same time, it takes a few days of a constant antibiotic level to get the infection under control.