2The dose shown is during the first week of life. The daily dose should be increased by approximately 33–50% after the first week of life. The lower dosage range should be used for neonates weighing less than 2 kg. After the first month of life, pediatric doses may be used. 3Dose is based on piperacillin component.
Except for amoxicillin, oral penicillins should be given 1–2 hours before or after a meal; they should not be given with food to minimize binding to food proteins and acid inactivation. Amoxicillin may be given without regard to meals. Blood levels of all penicillins can be raised by simultaneous administration of probenecid, 0.5 g (10 mg/kg in children) every 6 hours orally, which impairs renal tubular secretion of weak acids such as β-lactam compounds. Penicillins, like all antibacterial antibiotics, should never be used for viral infections and should be prescribed only when there is reasonable suspicion of, or documented infection with, susceptible organisms.
A. Penicillin
Penicillin G is a drug of choice for infections caused by streptococci, meningococci, some enterococci, penicillin-susceptible pneumococci, staphylococci confirmed to be non-β-lactamaseproducing, Treponema pallidum and certain other spirochetes, some Clostridium species, Actinomyces and certain other Gram-positive rods, and non-β-lactamase-producing Gram-negative anaerobic organisms. Depending on the organism, the site, and the severity of infection, effective doses range between 4 and 24 million units per day administered intravenously in four to six divided doses. High-dose penicillin G can also be given as a continuous intravenous infusion.
Penicillin V, the oral form of penicillin, is indicated only in minor infections because of its relatively poor bioavailability, the need for dosing four times a day, and its narrow antibacterial spectrum. Amoxicillin (see below) is often used instead.
Benzathine penicillin and procaine penicillin G for intramuscular injection yield low but prolonged drug levels. A single intramuscular injection of benzathine penicillin, 1.2 million units, is effective treatment for β-hemolytic streptococcal pharyngitis. Given intramuscularly once every 3–4 weeks, it prevents reinfection. Benzathine penicillin G, 2.4 million units intramuscularly once a week for 1–3 weeks, is effective in the treatment of syphilis. Procaine penicillin G was once a commonly used treatment for pneumococcal pneumonia and gonorrhea; however, it is rarely used now because many gonococcal strains are penicillin-resistant, and many pneumococci require higher doses of penicillin G or the use of more potent β-lactams.
B. Penicillins Resistant to Staphylococcal Beta-Lactamase (Methicillin, Nafcillin, and Isoxazolyl Penicillins)
These semisynthetic penicillins are indicated for infections caused by β-lactamase-producing staphylococci, although penicillin-susceptible strains of streptococci and pneumococci are also susceptible to these agents. Listeria monocytogenes, enterococci, and methicillin-resistant strains of staphylococci are resistant. In recent years the empirical use of these drugs has decreased substantially because of increasing rates of methicillin resistance in staphylococci. However, for infections caused by methicillin-susceptible and penicillin-resistant strains of staphylococci, these are considered drugs of choice.