Penicillin cures a wide range of bacterial infections, from common ones like strep throat and ear infections to serious conditions like syphilis, bacterial meningitis, and pneumonia. It remains one of the most prescribed antibiotics in the world nearly a century after its discovery, though antibiotic resistance has narrowed its effectiveness against certain bacteria.
Infections Penicillin Treats
Penicillin is used to treat infections caused by bacteria that are susceptible to it. The most common include:
- Strep throat: Penicillin is the first-line treatment. A typical course lasts 10 days and clears the infection in most people.
- Syphilis: Injectable penicillin is the preferred treatment at all stages of this sexually transmitted infection. No other antibiotic has matched its reliability here, and the CDC continues to recommend it as the standard of care.
- Ear infections (otitis media): Particularly in children, penicillin-class antibiotics are a go-to option.
- Pneumonia: Certain types of bacterial pneumonia, especially those caused by susceptible strains of the pneumococcus bacterium, respond well to penicillin.
- Urinary tract infections
- Lyme disease
- Tooth infections
- Meningitis: When bacterial meningitis is confirmed to be caused by the meningococcal bacterium, penicillin is a recommended treatment, provided testing shows the bacteria are fully susceptible.
- Salmonella infections
Penicillin works against many types of bacteria, including common strep and staph species, as well as certain bacteria responsible for gonorrhea and intestinal infections. That said, not every strain of these bacteria remains vulnerable to penicillin, which is why doctors sometimes choose a different antibiotic or test the bacteria first.
How Penicillin Kills Bacteria
Bacteria are surrounded by a rigid cell wall that keeps them from bursting. Building that wall requires a final construction step: cross-linking the wall’s structural material (called peptidoglycan) into a strong mesh. Penicillin mimics one of the building blocks involved in that step, tricking the bacterial enzyme into binding with the drug instead. Once bound, the enzyme is permanently disabled. Without proper cross-linking, the cell wall weakens and the bacterium ruptures and dies.
This is why penicillin only works on bacteria. Human cells don’t have cell walls, so the drug has no target in your own body. It’s also why penicillin does nothing against viruses like the flu or the common cold, since viruses lack cell walls entirely.
Two Forms of Penicillin
Penicillin comes in two main forms, and which one you receive depends on the infection being treated. Penicillin V (sometimes called phenoxymethyl penicillin) is the oral version, taken as a pill or liquid. It’s acid-stable, meaning it survives stomach acid and gets absorbed through the digestive tract. This is the form typically prescribed for strep throat, mild ear infections, and dental infections.
Penicillin G is given by injection, either into a muscle or through an IV. It’s used for more serious infections like syphilis, meningitis, and certain types of pneumonia where higher, more reliable drug levels in the bloodstream are needed.
Preventing Infections, Not Just Treating Them
Penicillin isn’t only used to cure active infections. It’s also prescribed as a preventive measure in certain situations. The most well-known example is rheumatic fever prevention. If someone has had rheumatic fever (a serious inflammatory condition triggered by untreated strep throat), they may take penicillin regularly to prevent strep infections from triggering another episode. This prophylaxis typically involves either an injection every three to four weeks or a daily oral dose, and it usually continues at least until age 21.
What Penicillin No Longer Cures Reliably
Antibiotic resistance has reduced penicillin’s reach significantly. The most notable example is Staphylococcus aureus, the bacterium behind many skin infections and hospital-acquired infections. Over 80% of staph isolates now produce an enzyme that breaks down penicillin, rendering the drug useless against them. This is why doctors typically prescribe different antibiotics for staph infections unless lab testing confirms the specific strain is susceptible.
Resistance has also emerged in some strains of gonorrhea and pneumococcus. For infections where resistance is common, doctors often start with a broader-spectrum antibiotic or wait for culture results before deciding on treatment. Penicillin still works well against many bacteria, but it’s no longer the universal solution it once was.
Penicillin Allergies: Rarer Than You Think
About 10% of people in the United States report having a penicillin allergy. When those patients are formally evaluated by allergists, however, fewer than 1% turn out to be truly allergic. Many reported allergies stem from childhood reactions that were actually side effects (like a stomach upset or a rash from a viral illness) rather than genuine immune responses. This matters because a false allergy label can steer patients toward less effective or broader-spectrum antibiotics when penicillin would be the better choice. If you’ve been told you’re allergic but aren’t sure, allergy testing can clarify whether penicillin is safe for you.