Penicillins

Penicillins are among the most prescribed antibiotics—but they’re not all the same. Choosing the right one involves understanding the spectrum, side effects, and special precautions. This guide breaks it down for pharmacists and interns, ensuring clarity and clinical relevance.

General Principles of Link:

Anti-Infectives

Medicine Link

1. Amoxicillin

2. Amoxicillin + Clavulanic Acid

3. Dicloxacillin

4. Flucloxacillin

5. Phenoxymethylpenicillin (Penicillin V)

   
   

General Mode of Action

All penicillins are bactericidal:

• Bind to penicillin-binding proteins (PBPs)

• Inhibit bacterial cell wall synthesis

• Lead to bacterial lysis and death

  
  

Spectrum of Activity

Type	Coverage	Notes
Narrow	Gram-positive only	e.g., Penicillin V, Flucloxacillin – no MRSA cover
Moderate	Gram-positive + some Gram-negative	e.g., Amoxicillin – limited by beta-lactamase
Broad	+ Beta-lactamase producers	e.g., Amox/Clav, Pip/Tazo – cover Pseudomonas, Klebsiella

Comparative Overview

Drug	Spectrum	Route	Highlights
Amoxicillin	Moderate	Oral, IV	First-line for URTI, UTI, dental
Amox/Clav	Broad	Oral, IV	Better against beta-lactamase producers; more GI upset
Dicloxacillin	Narrow (Staph)	Oral	Penicillinase-resistant; skin infections
Flucloxacillin	Narrow (Staph)	Oral, IV	Alternative to dicloxacillin; watch for hepatic issues
Phenoxymethylpenicillin (Pen V)	Narrow	Oral	Tonsillitis, rheumatic fever prophylaxis
Piperacillin + Tazobactam	Broad	IV	Covers Pseudomonas, hospital infections

Detailed Drug Profiles

1. Amoxicillin

Amoxicillin

✅ Use: URTI, UTI, dental infections, endocarditis prophylaxis

💊 Dose: 250–500 mg q8h OR 1 g BD (oral); 1 g IV q6h

⚠️ Cautions: Adjust in renal impairment; mild rash common

🗣️ Counselling: Can take with or without food; finish full course

📦 PBS Brands: Amoxil®, Alphamox®, Amoxila®

2. Amoxicillin + Clavulanic Acid

Amoxicillin + Clavulanic Acid

✅ Use: Beta-lactamase infections (e.g. bites, hospital pneumonia)

💊 Dose: 500/125 or 875/125 mg q12h (oral); 1 g IV q8h

⚠️ Cautions: ↑ GI upset, cholestatic hepatitis risk in prolonged use

🗣️ Counselling: Take with food; report jaundice or prolonged GI upset

📦 PBS Brands: Augmentin Duo®, Curam®, AlphaClav®

3. Dicloxacillin

✅ Use: Mild staph skin infections

💊 Dose: 250–500 mg q6h

⚠️ Cautions: Cholestatic hepatitis risk; adjust in renal impairment

🗣️ Counselling: Take 30 min before or 2 hrs after food

📦 PBS Brands: Distapha®

4. Flucloxacillin

Flucloxacillin

✅ Use: Severe staph infections, pneumonia, osteomyelitis

💊 Dose: 250–500 mg q6h (oral); up to 2 g q6h IV

⚠️ Cautions: Hepatotoxicity risk ↑ if >2 weeks or in older adults

🗣️ Counselling: Take on empty stomach; monitor LFTs if long course

📦 PBS Brands: Flopen®, Staphylex®, Flucil®

5. Phenoxymethylpenicillin (Penicillin V)

henoxymethylpenicillin

✅ Use: S. pyogenes (tonsillitis), rheumatic fever prevention

💊 Dose: 250–500 mg q6h

⚠️ Cautions: Adjust in renal impairment; low risk of rash

🗣️ Counselling: Take regularly; finish 10-day course for strep throat

📦 PBS Brands: Cilicaine VK®, Aspecillin VK®

⚠️ Key Precautions

Risk	Advice
Allergy	Always ask about past reactions; high cross-reactivity with other β-lactams
Renal Impairment	Dose adjustments needed; risk of neurotoxicity at high doses
Sodium Load	IV forms contain sodium – caution in HF or salt-restricted diets
Hepatic Function	Flucloxacillin, Dicloxacillin – monitor LFTs for prolonged use
Cross-Reactivity	Cephalosporins/carbapenems – use caution in known allergy cases

Adverse Effects Overview

Warning: Potential adverse effects of penicillin include neurological issues, liver function test abnormalities, and rashes.

Frequency	Effects
Common	Rash, diarrhoea, nausea, local pain (injection)
Infrequent	C. difficile, vomiting
Rare	Seizures (high dose), electrolyte disturbance, SJS/TEN, cholestatic hepatitis (especially flucloxacillin)

Auxe Tips

• Dosing: Frequent dosing is key for time-dependent antibiotics like penicillins

• Monitoring: FBC, renal, hepatic tests for long-term or high-dose regimens

• IV Safety: Don’t mix with aminoglycosides—flush lines separately

• Counselling: Emphasise allergy history, hydration, GI tolerance

• Hepatotoxicity: More likely in flucloxacillin (esp. >2 weeks, older adults)

  
  

Community Pharmacy Counselling Points

Point	Advice
GI Upset	Amox/Clav more likely → take with food
Finish Course	Stress full-course adherence even if symptoms improve
Empty Stomach	Dicloxacillin, Flucloxacillin best absorbed pre-meal
Allergy Signs	Rash, swelling, breathing issues → stop and seek help
Breastfeeding	Safe, but may cause mild diarrhoea in infants

Final Thoughts from Auxe Pharma

Penicillins remain one of the safest and most effective antibiotics we have—but choosing the right one matters.

As an intern pharmacist:

• Know the spectrum

• Recognise adverse effect patterns

• Educate patients on adherence and precautions

• Always screen for allergies and hepatic/renal issues

Your confidence and accuracy in managing these common antibiotics can greatly influence safe prescribing and improved patient outcomes.