Antibiotics are frequently prescribed in hospitalized and in outpatients. We review four important aspects for their daily prescription. In elderly patients, the prescription should take into account changes in the volume of distribution and the usual decline in renal function even in the absence of chronic kidney disease. Particular antibiotics can trigger infection with Clostridium difficile. We discuss actual and novel strategies for its prevention. Renal toxicity of antibiotics includes acute tubular necrosis, interstitial nephritis and crystals obstructing renal tubule, depending on the class of antibiotics. Plasmatic levels of particular antibiotics should be measured either at trough or at peak levels depending of the antibiotic prescribed.