Narrow spectrum antibiotics are needed for the infection when the causative organism is known and will not kill as many as of the normal microorganisms in the body as the broad spectrum antibiotics.when the bacterial spectrum of chloramphenicol and the first tetracyclines could be strikingly opposed to the narrow spectrum of activities of penicillin G, and streptomycin. aminopenicillins, then ureidopenicillins, became the broad-spectrum penicillins in comparison with penicillin G. Until then, the quality of being broad spectrum or narrow spectrum was given to an antibiotic only when referring to a comparator. Later, the reference to a comparator was omitted, and broad and narrow lost their relativities and became independent characteristics of a compound, used with different meaning and often improperly. Broad spectrum as an expression of greater therapeutic security has mainly been used by the pharmaceutical industry. Most antibiotics are prescribed empirically on a presumptive diagnosis. This implies that several bacterial species are possible causes of the disease. It is correct that a broad-spectrum antibiotic may offer a better chance of covering the causative microorganisms. In the same way, a broad-spectrum agent is indicated in a large number of clinical situations.