Penicillin G is a broad-spectrum, beta-lactam naturally occurring penicillin antibiotic with antibacterial activity. Penicillin G binds to and inactivates the penicillin binding proteins (PBPs) located inside the bacterial cell wall. Inactivation of PBPs interferes with the cross-linkage of peptidoglycan chains necessary for bacterial cell wall strength and rigidity. This interrupts bacterial cell wall synthesis and results in the weakening of the bacterial cell wall and eventually causing cell lysis.

Penicillin G is narrow spectrum antibiotic used to treat infections caused by susceptible bacteria. It is a natural penicillin antibiotic that is administered intravenously or intramuscularly due to poor oral absorption. Penicillin G may also be used in some cases as prophylaxis against susceptible organisms. Natural penicillins are considered the drugs of choice for several infections caused by susceptible gram positive aerobic organisms, such as Streptococcus pneumoniae, groups A, B, C and G streptococci, nonenterococcal group D streptococci, viridans group streptococci, and non-penicillinase producing staphylococcus. Aminoglycosides may be added for synergy against group B streptococcus (S. agalactiae), S. viridans, and Enterococcus faecalis. The natural penicillins may also be used as first or second line agents against susceptible gram positive aerobic bacilli such as Bacillus anthracis, Corynebacterium diphtheriae, and Erysipelothrix rhusiopathiae. Natural penicillins have limited activity against gram negative organisms; however, they may be used in some cases to treat infections caused by Neisseria meningitidis and Pasteurella. They are not generally used to treat anaerobic infections. Resistance patterns, susceptibility and treatment guidelines vary across regions.