Here is your full GP-level deep dive on Penicillins - Day 1 of your antibiotic revision series.
💊 Day 1 - Penicillins: The Complete GP Revision
Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics | Katzung's Basic & Clinical Pharmacology 16e | CDC Antibiotic Stewardship Report 2025 | IDSA AMR Guidance 2026
1. CLASSIFICATION AT A GLANCE
| Class | Key Drugs | Spectrum |
|---|
| Natural penicillins | Penicillin G (IV), Penicillin V (oral), Benzathine Pen G (IM) | Narrow - Gram+, spirochetes |
| Antistaphylococcal | Oxacillin, Nafcillin, Dicloxacillin | Narrow - MSSA only |
| Aminopenicillins | Ampicillin, Amoxicillin | Broader - some Gram- added |
| + β-lactamase inhibitor | Amoxicillin-Clavulanate (Co-amoxiclav), Ampicillin-Sulbactam | Broad - overcomes resistance |
| Antipseudomonal | Piperacillin-Tazobactam (Pip-Taz) | Extended Gram- + Pseudomonas |
2. MECHANISM OF ACTION
Penicillins are β-lactam antibiotics - they are bactericidal and work by:
- Irreversibly binding to Penicillin-Binding Proteins (PBPs) on the bacterial cell membrane
- PBPs are transpeptidase enzymes responsible for the final cross-linking step of peptidoglycan (the structural backbone of the bacterial cell wall)
- Inhibiting PBPs leads to accumulation of defective cell wall precursors → the bacteria cannot maintain osmotic integrity → cell lysis and death
Key point: They are time-dependent killers. Efficacy depends on maintaining drug concentration above the MIC for >40-50% of the dosing interval (not on peak concentration).
3. PHARMACOKINETICS - DEEP DIVE
| Parameter | Details |
|---|
| Absorption | Amoxicillin - good oral bioavailability (~80%), can be taken with food. Ampicillin and Pen V - 30-55%, must be given on empty stomach (1-2 hrs before or after meals) |
| Distribution | Widely distributed into tissues, joint/pleural/pericardial fluid, bile. Poor penetration into: CSF (normal meninges <1% of plasma), prostate, phagocytic cells, intraocular fluid. CSF penetration improves to ~5% with meningeal inflammation |
| Protein binding | Variable. Nafcillin/Dicloxacillin - highly protein-bound. Amoxicillin/Ampicillin - lower binding |
| Metabolism | Minimal hepatic metabolism. Nafcillin is the exception - primarily biliary excretion |
| Elimination | Predominantly renal - glomerular filtration AND tubular secretion. Short half-life: 30-90 minutes for most penicillins |
| Dose adjustment | Reduce dose in significant renal impairment (except Nafcillin) |
| Probenecid interaction | Blocks renal tubular secretion of penicillins → prolongs half-life and raises drug levels. Still used occasionally for gonorrhoea or syphilis regimens |
- Goodman & Gilman's, p.1170
4. SPECTRUM OF ACTIVITY
Natural Penicillins (Pen G, Pen V, Benzathine Pen G)
- Gram-positive: Most streptococci (S. pyogenes, viridans group), penicillin-susceptible S. pneumoniae, non-β-lactamase staphylococci
- Gram-negative: Neisseria meningitidis (still very sensitive), some anaerobes (Clostridium spp., Actinomyces)
- Spirochetes: T. pallidum (syphilis), Borrelia, Leptospira - remain highly sensitive
- ⚠️ >90% of S. aureus is now resistant to natural penicillins due to β-lactamase production
- ⚠️ Penicillin-resistant S. pneumoniae is common in paediatric populations
Aminopenicillins (Amoxicillin, Ampicillin)
- All of the above, PLUS:
- H. influenzae (non-β-lactamase strains), E. coli, Proteus mirabilis, Salmonella, Enterococcus faecalis
- Amoxicillin is the most active oral β-lactam against S. pneumoniae with elevated MICs - use higher doses (80-90 mg/kg/day) for AOM in children
- ⚠️ Ampicillin-resistant H. influenzae widespread - use Co-amoxiclav instead
- ⚠️ No activity against Klebsiella, Enterobacter, Pseudomonas, Serratia, Citrobacter (intrinsic β-lactamases)
Amoxicillin-Clavulanate (Co-amoxiclav)
- Adds coverage for β-lactamase-producing H. influenzae, Moraxella catarrhalis, E. coli, Klebsiella, S. aureus (MSSA)
- Does NOT cover MRSA, Pseudomonas, Enterobacter
Piperacillin-Tazobactam
-
Extended Gram-negative coverage including Pseudomonas aeruginosa, most Enterobacterales (including some ESBL-producers), anaerobes
-
A hospital-level drug; not for GP outpatient prescribing typically
-
Katzung 16e, p.1244-1245
5. GP-RELEVANT CLINICAL INDICATIONS + DOSES
Amoxicillin
| Indication | Adult Dose | Duration | Notes |
|---|
| Streptococcal pharyngitis | 500 mg TDS or 1g BD | 5-7 days | Still first-line; penicillin V is an option |
| Community-acquired pneumonia (mild, low-risk) | 1g TDS | 5 days | For suspected pneumococcal; use with azithromycin if atypicals suspected |
| Acute otitis media | 500 mg TDS | 5-7 days | High-dose 80-90 mg/kg/day in children |
| Acute sinusitis (bacterial) | 500 mg TDS | 5 days | Only for bacterial, not viral |
| H. pylori eradication | As part of triple/quadruple therapy | 10-14 days | See regimens below |
Amoxicillin-Clavulanate (Co-amoxiclav)
| Indication | Adult Dose | Duration |
|---|
| Otitis media (treatment failure/severe) | 625 mg TDS or 1g BD | 5-7 days |
| Exacerbation of chronic bronchitis | 625 mg TDS | 5 days |
| Sinusitis (with β-lactamase risk) | 625 mg BD/TDS | 5 days |
| UTI (Co-amoxiclav, second-line) | 625 mg TDS | 7 days |
| Animal bites | 625 mg TDS | 5-7 days |
| Cellulitis (mild, community-acquired) | 625 mg TDS | 5-7 days |
2025-2026 Update (Wales/UK): National guidance now targets
≥75% of amoxicillin prescriptions to be 5-day courses rather than 7 days. Evidence shows equivalent clinical outcomes with reduced antibiotic exposure and AMR risk. (
GOV.Wales AMR 2025-2027)
Benzathine Penicillin G (IM)
| Indication | Dose | Schedule |
|---|
| Streptococcal pharyngitis | 1.2 million units IM | Single dose |
| Syphilis (primary/secondary/latent early <1yr) | 2.4 million units IM | Single dose |
| Syphilis (late latent/>1yr) | 2.4 million units IM | Weekly x 3 weeks |
| Rheumatic fever prophylaxis | 1.2 million units IM | Every 3-4 weeks |
H. pylori Eradication (Amoxicillin component)
Standard first-line Bismuth Quadruple Therapy:
- PPI + Bismuth + Tetracycline + Metronidazole x 14 days
- If bismuth unavailable, Non-bismuth concomitant therapy (PPI + Amoxicillin + Clarithromycin + Metronidazole x 14 days) - recent 2026 meta-analysis (PMID: 41498306) confirms high eradication rates with this regimen.
6. RESISTANCE MECHANISMS - THE "BIG 3"
| Mechanism | Details | Clinical Example |
|---|
| β-lactamase production | Most common. Enzyme cleaves the β-lactam ring | S. aureus, H. influenzae, E. coli. Overcome with clavulanate/sulbactam/tazobactam |
| Altered PBPs | PBP mutations reduce penicillin binding affinity | MRSA (PBP2a - mecA gene); Penicillin-resistant S. pneumoniae. β-lactamase inhibitors do NOT help here |
| Efflux pumps / Reduced permeability | Drug pumped out or can't enter the cell | Gram-negative bacteria (P. aeruginosa); combined with β-lactamase |
2026 IDSA AMR Guidance note: ESBLs (Extended-Spectrum β-Lactamases) inactivate most penicillins, cephalosporins, and aztreonam. ESBL-producing Enterobacterales generally remain susceptible to carbapenems. (
IDSA AMR Guidance 2026)
7. ADVERSE EFFECTS
| Effect | Frequency | Details |
|---|
| Hypersensitivity reactions | Most important | Ranges from maculopapular rash (~5%) to urticaria, serum sickness, to life-threatening anaphylaxis (<0.05%) |
| GI disturbance | Common with Co-amoxiclav | Nausea, diarrhoea - especially clavulanate component. Take with food for Co-amoxiclav |
| C. difficile | Less common than clindamycin/FQ | Still a risk, especially in elderly/hospitalized |
| Neurotoxicity (rare) | High-dose IV | Seizures with very high doses (e.g., >20 MU/day penicillin G), especially in renal impairment |
| Haematological | Rare | Haemolytic anaemia, neutropaenia, thrombocytopaenia with prolonged use |
| Electrolyte disturbance | IV formulations | High-dose IV piperacillin can cause hypokalaemia |
8. PENICILLIN ALLERGY - THE UPDATED PICTURE ⭐ (Critical GP Pearl)
This is one of the most clinically important and commonly mismanaged issues:
- ~10% of patients self-report penicillin allergy
- <1% are truly allergic (CDC 2025)
- Most "allergies" are:
- Maculopapular rash from amoxicillin during EBV infection (misattributed allergy)
- Childhood GI side effects
- Family history
Cross-reactivity with Cephalosporins:
- Overall cross-reactivity: ~2-7% (likely an overestimate based on older data)
- True cross-reactivity is R-group side chain dependent, not just ring structure
- Patients with penicillin allergy + cephalosporin with different R-group: cross-reactivity <2%
- Patients with history of anaphylaxis to penicillin: cross-reactivity up to 50% - skin test or use alternative
- Penicillin vs. carbapenem: <1%
- Penicillin vs. monobactam (aztreonam): 0% (except aztreonam-ceftazidime share R-group)
GP action: When you see "penicillin allergy" on a patient's record - clarify the reaction. Encourage de-labelling of patients with low-risk reactions. An inaccurate penicillin allergy label forces use of broader-spectrum antibiotics, driving resistance.
- Washington Manual of Medical Therapeutics
9. KEY DRUG INTERACTIONS
| Drug | Interaction | Clinical Significance |
|---|
| Probenecid | Blocks tubular secretion → raises penicillin levels | Used intentionally for syphilis/gonorrhoea regimens |
| Warfarin / Anticoagulants | Broad-spectrum penicillins alter gut flora → reduce Vit K → potentiate anticoagulation | Monitor INR |
| Oral contraceptives | Theoretical reduction in efficacy (old data, largely dismissed) | Advise per local guidance |
| Methotrexate | Penicillins reduce renal MTX excretion → toxicity risk | Avoid combination or monitor MTX levels closely |
| Allopurinol | Increases incidence of ampicillin rash | Rash rate rises from 5% to ~20% |
10. PRESCRIBING PEARLS FOR GPS 🩺
- Amoxicillin with food - always fine. All other oral penicillins - empty stomach
- Amoxicillin for CAP: 1g TDS is now recommended over 500mg TDS due to rising pneumococcal MICs
- Co-amoxiclav for bites - ALWAYS the first choice for human/animal bites
- Benzathine penicillin for syphilis - never substitute with oral penicillin (incomplete absorption)
- Don't use ampicillin empirically for UTI or typhoid - resistance rates too high
- Shorter courses work: 5-day amoxicillin is as effective as 7 days for most community infections (2025-2026 guideline shift)
- Penicillin allergy de-labelling is a stewardship priority - most self-reported allergies are not true allergies
- MRSA coverage: No penicillin covers MRSA. If MRSA suspected, escalate
- Enterococcal endocarditis: Ampicillin + Ceftriaxone (preferred over Ampicillin + Gentamicin - same efficacy, less nephrotoxicity)
- Renal dose adjustment: Required for most penicillins except Nafcillin (biliary excretion)
11. MEMORY AID
PENICILLINS = "BLAST the Cell Wall"
B - Bactericidal (cell wall synthesis inhibitor)
L - β-Lactam ring is the pharmacophore
A - Allergy is the #1 concern (anaphylaxis possible)
S - Spectrum varies by subclass (narrow → broad)
T - Time-dependent killing (T > MIC is key PK/PD)
Quick Recall Card
| Natural Pen | Aminopenicillins | Co-amoxiclav | Antistaphylococcal |
|---|
| MRSA? | No | No | No | No (only MSSA) |
| Pseudomonas? | No | No | No | No |
| H. influenzae? | No | Some strains | Yes | No |
| S. aureus (MSSA)? | No (resist) | No | Yes | Yes (DOC) |
| Streptococci? | Yes (DOC) | Yes | Yes | Yes |
| Spirochetes? | Yes (DOC) | Yes | Yes | No |
That's Day 1 - Penicillins - fully covered from mechanism to GP pearls with 2025-2026 updates.
Tomorrow's suggestion: Cephalosporins (natural progression from penicillins, building on β-lactam knowledge) - or tell me which class you'd prefer next! 🎯
