Effectiveness In vivo antibiotic effectiveness is affected by many factors, including Pharmacokinetics: The time course of antibiotic levels, which are affected by factors such as absorption, distribution (concentration in fluids and tissues, protein binding), rate of metabolism, and excretion Pharmacodynamics: The antimicrobial activity of local antibiotic concentrations on the target pathogen and that pathogen's response including resistance Presence of foreign materials Control of source of infection Drug interactions or inhibiting substances Host defense mechanisms Bactericidal drugs kill bacteria.