How quickly do antibiotics start to work in your body?

Depends. The speed of antibiotics varies with the route of administration (intravenous or injected are faster than oral), the type of antibiotic (penicillin works faster than erythromycin) and the rate the germ reproduces. Strep grows fast so meds that interfere with reproduction kill quickly. Tuberculosis grows slowly so meds seem to take forever to kill it.