The aminoglycosides are natural, as well as semisynthetic, bactericidal antibiotics. They are considered to be narrow-spectrum as they are usually used against gram-negative bacteria, but they are also effective against some gram-positive strains. Parenteral administration of these drugs provides good absorption, but they have poor oral absorption. If they are given orally, they are poorly absorbed from the gastrointestinal tract and thus are used to cleanse the tract before bowel surgery. The aminoglycosides are reserved to treat serious infections as they can produce powerful side effects. Serum drug levels must be checked frequently as there is a small difference between toxic and safe levels. Their distribution is mostly to extracellular fluid, and they are eliminated by the kidney.