Penicillin: Classification, Mechanism, Uses, Adverse Effects, Dosage, Contraindications & Drug Interactions

Penicillin: Classification, Mechanism, Uses, Adverse Effects, Dosage, Contraindications & Drug Interactions

Classification of Penicillin

1. Natural Penicillin

Benzyl penicillin (Penicillin G) — parenteral
Penicillin V — oral

2. Semisynthetic Penicillin

Penicillin V (acid stable)
Cloxacillin (penicillinase-resistant)
Ampicillin, Amoxicillin (aminopenicillins — broad spectrum)
Piperacillin (antipseudomonal)

3. β-lactamase inhibitors (used in combination)

Clavulanic acid
Sulbactam
Tazobactam

Mechanism of Action

Inhibit bacterial cell wall synthesis

Bind to Penicillin-Binding Proteins (PBPs)
Inhibit transpeptidase enzyme → prevent cross-linking of peptidoglycan strands
Leads to weak cell wall → osmotic lysis

Bactericidal (only in actively growing bacteria)
Time-dependent killing

Natural Penicillin: Benzyl Penicillin (Penicillin G)

Spectrum: Gram-positive cocci, Gram-negative cocci, some spirochetes (e.g., Treponema pallidum)
Uses:

Streptococcal infections, meningococcal meningitis
Syphilis
Gas gangrene (Clostridium perfringens)
Diphtheria (adjunct)

Semisynthetic Penicillin

Penicillin V

Acid-stable, oral use, mainly mild streptococcal infections

Cloxacillin

Resistant to staphylococcal β-lactamase → used in MSSA infections

Ampicillin / Amoxicillin

Extended spectrum → Gram-negative bacilli (e.g., H. influenzae, E. coli, Salmonella)
Amoxicillin: better oral absorption, used in URTI, otitis media, dental infections

Piperacillin

Antipseudomonal, used in serious Pseudomonas aeruginosa infections (often with tazobactam)

β-lactamase Inhibitors

Clavulanic acid, Sulbactam, Tazobactam

No significant antibacterial action alone
Irreversibly bind bacterial β-lactamase → protect penicillin from degradation
Examples of combinations:

Amoxicillin + clavulanic acid (Augmentin)
Piperacillin + tazobactam
Ampicillin + sulbactam

Therapeutic Uses (Summary Table)

Drug	Main Uses
Benzyl penicillin	Syphilis, streptococcal infections, meningitis
Penicillin V	Mild strep throat, dental infections
Cloxacillin	MSSA skin & soft tissue infections
Ampicillin	Meningitis (with gentamicin), enterococcal infections
Amoxicillin	URTI, otitis media, prophylaxis in rheumatic fever
Piperacillin	Pseudomonas infections (hospital-acquired)

Adverse Effects

Hypersensitivity reactions (rash → anaphylaxis)
GI upset, diarrhea
Nephritis (rare)
Seizures (high dose, especially in renal failure)
Superinfection (e.g., C. difficile)
Jarisch–Herxheimer reaction (in syphilis treatment)

Dosage (General Ranges)

Benzyl penicillin: 1–4 million units IV every 4–6 hrs
Penicillin V: 250–500 mg orally every 6–8 hrs
Cloxacillin: 250–500 mg orally every 6 hrs
Ampicillin: 250–500 mg orally every 6 hrs / 1–2 g IV every 4–6 hrs
Amoxicillin: 250–500 mg orally every 8 hrs
Piperacillin/tazobactam: 3.375–4.5 g IV every 6–8 hrs

Contraindications

History of severe hypersensitivity to β-lactams
Caution in:

Renal impairment
Seizure disorders
Infectious mononucleosis (↑ rash with ampicillin/amoxicillin)

Drug Interactions

Probenecid: ↑ penicillin levels (↓ renal excretion)
Methotrexate: ↓ clearance → toxicity risk
Aminoglycosides: Synergistic effect (but physically incompatible in same IV line)
Tetracyclines: May antagonize penicillin bactericidal effect
Oral contraceptives: Possible ↓ efficacy (controversial)
Warfarin: Some penicillins alter gut flora → ↑ INR

Memory Tips

“Pen G → Give in G(ross) infections” (serious infections)
“V → Very acid-stable” (oral)
“Cloxa → Clocks staph” (β-lactamase resistant)
“Amo & Amp → Ample spectrum” (broad Gram coverage)
“Piper → Pseudomonas Piper”
Clav/Sulb/Tazo → “Clubs the β-lactamase”

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