Benzylpenicillin, discovered by Alexander Fleming in 1928 from the mold Penicillium notatum, was one of the first antibiotics used to treat bacterial infections. Developed further by Howard Florey and Ernst Chain, it became widely available in the 1940s for serious infections like strep and staph. Benzylpenicillin works by disrupting bacterial cell wall formation, killing the bacteria and stopping infection.
BRAND NAMES
Some common brand names for benzylpenicillin (penicillin G) include:
Pfizerpen – injectable penicillin G
Bicillin – long-acting (benzathine or procaine penicillin)
Pentids – injectable formulations
Wycillin – benzathine penicillin G
MECHANISM OF ACTION
Benzylpenicillin works by blocking bacterial cell wall synthesis. It binds to enzymes called penicillin-binding proteins, preventing proper cross-linking of the cell wall. This weakens the wall, causing the bacteria to burst and die, making benzylpenicillin bactericidal, especially against actively dividing Gram-positive bacteria.
PHARMACOKINETICS
Absorption
Benzylpenicillin is poorly absorbed orally, so it is usually given by injection. After administration, it spreads quickly in body fluids and tissues, reaching most sites of infection.
Distribution
Benzylpenicillin is widely distributed in the body, reaching most tissues and fluids, including the kidneys, liver, and lungs. It penetrates the cerebrospinal fluid only when the meninges are inflamed, and its concentration is lower in the eye and prostate.
Metabolism
Benzylpenicillin is minimally metabolized in the body. Most of the drug remains in its active form and is eventually excreted unchanged by the kidneys.
Excretion
Benzylpenicillin is primarily excreted unchanged by the kidneys through glomerular filtration and tubular secretion. Small amounts may also be eliminated in bile. Its elimination is faster in patients with normal kidney function and slower in those with renal impairment.
PHARMACODYNAMICS
Benzylpenicillin is a bactericidal antibiotic that kills bacteria by inhibiting cell wall synthesis. Its effectiveness depends on the time the drug concentration remains above the minimum inhibitory concentration (MIC) for the target bacteria, making it time-dependent. It works best against actively dividing Gram-positive bacteria and some Gram-negative cocci.
ADMINISTRATION
Benzylpenicillin is usually given by injection either intravenously (IV) or intramuscularly (IM) because it is poorly absorbed orally.
DOSAGE AND STRENGTH
Intravenous (IV): 2–24 million units per day, divided every 4–6 hours for severe infections.
Intramuscular (IM): 600,000–1.2 million units every 4–12 hours for milder infections.
FOOD INTERACTIONS
Benzylpenicillin has no significant food interactions because it is usually given by injection (IV or IM) rather than taken orally. Therefore, food does not affect its absorption or effectiveness.
DRUG INTERACTIONS
Benzylpenicillin may interact with certain drugs. Probenecid can increase its levels by reducing renal excretion. Tetracyclines may reduce its effectiveness by inhibiting bacterial growth. It can also enhance the effect of oral anticoagulants, increasing the risk of bleeding.
CONTRAINDICATIONS
Benzylpenicillin is contraindicated in patients with a known hypersensitivity to penicillins or a history of severe allergic reactions (such as anaphylaxis) to β-lactam antibiotics. It should be used with caution in individuals with a history of allergies or asthma.
SIDE EFFECTS
Common side effects of benzylpenicillin include allergic reactions such as rash, itching, and fever. More serious effects may include anaphylaxis, which is rare but life-threatening. Other side effects can include nausea, vomiting, diarrhea, and pain at the injection site.
TOXICITY
Benzylpenicillin toxicity is rare but may occur with high doses or impaired kidney function. It can lead to neurotoxic effects such as seizures, confusion, and muscle twitching. Electrolyte imbalances (due to sodium or potassium salts) and hypersensitivity reactions may also occur in severe cases.
