MJDF Pharmacology and Therapeutics — Questions and Answers
Question 1: What is the maximum recommended dose of lidocaine with 1:80,000 adrenaline for a healthy adult weighing 70 kg, and how many 2.2 mL cartridges does this equate to?
- 1 cartridge (44 mg lidocaine)
- 7 cartridges (approximately 308 mg lidocaine) — based on the maximum dose of 4.4 mg/kg for lidocaine with adrenaline (Correct answer)
- 20 cartridges (880 mg lidocaine)
- 3 cartridges (132 mg lidocaine)
Correct answer: 7 cartridges (approximately 308 mg lidocaine) — based on the maximum dose of 4.4 mg/kg for lidocaine with adrenaline
The maximum dose of lidocaine with adrenaline is 4.4 mg/kg (or 7 mg/kg in some references, though 4.4 mg/kg is the commonly cited UK maximum). For a 70 kg adult: 4.4 × 70 = 308 mg. Each 2.2 mL cartridge of 2% lidocaine contains 44 mg lidocaine (2% = 20 mg/mL × 2.2 mL = 44 mg). Therefore: 308/44 = 7 cartridges. In practice, this is a theoretical maximum — most dental procedures require far fewer cartridges, and dose should always be tailored to the individual patient's weight, age, and medical status.
Question 2: What is the mechanism of action of amoxicillin and why is it the first-line antibiotic for most dental infections?
- Amoxicillin inhibits protein synthesis at the 30S ribosomal subunit
- Amoxicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (transpeptidases), preventing peptidoglycan cross-linking; it is first-line because it has a broad spectrum covering most oral pathogens, good oral bioavailability (approximately 90%), and a well-established safety profile (Correct answer)
- Amoxicillin disrupts bacterial DNA replication
- Amoxicillin inhibits folic acid synthesis in bacteria
Correct answer: Amoxicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (transpeptidases), preventing peptidoglycan cross-linking; it is first-line because it has a broad spectrum covering most oral pathogens, good oral bioavailability (approximately 90%), and a well-established safety profile
Amoxicillin belongs to the aminopenicillin group of beta-lactam antibiotics. It binds to penicillin-binding proteins (PBPs/transpeptidases) in the bacterial cell wall, inhibiting the final cross-linking step of peptidoglycan synthesis. This weakens the cell wall, leading to osmotic lysis and bacterial death (bactericidal). It is first-line for dental infections because: it covers the predominant oral pathogens (streptococci, some anaerobes), has excellent oral absorption (approximately 90% bioavailability), achieves good tissue concentrations, and has a well-established safety record. Metronidazole is added for severe infections to enhance anaerobic coverage.
Question 3: A patient allergic to penicillin requires antibiotics for an acute dental abscess. What is the recommended alternative according to FGDP/BNF guidelines?
- Prescribe amoxicillin anyway as cross-reactivity is minimal
- Metronidazole 400 mg three times daily for 5 days (if the allergy is to all beta-lactams) or clarithromycin 250-500 mg twice daily for 5 days; clindamycin is reserved for severe infections due to the risk of Clostridium difficile colitis (Correct answer)
- Tetracycline 250 mg four times daily for 14 days
- No antibiotics can be given to penicillin-allergic patients
Correct answer: Metronidazole 400 mg three times daily for 5 days (if the allergy is to all beta-lactams) or clarithromycin 250-500 mg twice daily for 5 days; clindamycin is reserved for severe infections due to the risk of Clostridium difficile colitis
For patients with confirmed penicillin allergy, alternatives for dental infections include: metronidazole 400 mg TDS for 5 days (effective against obligate anaerobes common in dental abscesses — note: does NOT cover aerobic streptococci when used alone), or clarithromycin 250-500 mg BD for 5 days (macrolide — covers streptococci but resistance is increasing). Clindamycin 300 mg QDS is effective but reserved for severe infections due to the risk of pseudomembranous colitis (C. difficile). If the allergy is mild (non-anaphylactic), cephalosporins may be considered as cross-reactivity is approximately 1-2%.
Question 4: What is the analgesic ceiling effect and which commonly used dental analgesic demonstrates this property?
- It means that analgesics become more effective at higher doses without limit
- The analgesic ceiling is the dose above which no additional pain relief is achieved despite increasing the dose; paracetamol and NSAIDs (e.g., ibuprofen) demonstrate this — increasing beyond the recommended maximum dose does not improve analgesia but does increase the risk of adverse effects (Correct answer)
- Only opioid analgesics have a ceiling effect
- The ceiling effect means the drug stops working after repeated use
Correct answer: The analgesic ceiling is the dose above which no additional pain relief is achieved despite increasing the dose; paracetamol and NSAIDs (e.g., ibuprofen) demonstrate this — increasing beyond the recommended maximum dose does not improve analgesia but does increase the risk of adverse effects
The analgesic ceiling effect means there is a maximum dose beyond which no further pain relief is obtained, only increased toxicity. Paracetamol (maximum 4g/day for healthy adults) and NSAIDs (e.g., ibuprofen maximum 1.2g/day OTC, 2.4g/day prescribed) both have ceiling effects. Exceeding these doses increases the risk of hepatotoxicity (paracetamol) or GI bleeding, renal impairment, and cardiovascular events (NSAIDs) without additional analgesic benefit. Opioids (codeine, dihydrocodeine) have less defined ceilings for analgesia but are dose-limited by adverse effects (respiratory depression, sedation).
Question 5: What are the contraindications for using adrenaline-containing local anaesthetics in dental patients?
- Adrenaline-containing LA is contraindicated in all patients over 50 years old
- There are very few absolute contraindications; caution is needed with uncontrolled hyperthyroidism, phaeochromocytoma, and patients on non-selective beta-blockers (risk of hypertensive crisis); controlled cardiovascular disease is NOT a contraindication — the benefit of pain control outweighs the minimal cardiovascular risk of dental doses of adrenaline (Correct answer)
- Adrenaline is contraindicated in all patients with hypertension
- Adrenaline is contraindicated whenever a patient is taking any medication
Correct answer: There are very few absolute contraindications; caution is needed with uncontrolled hyperthyroidism, phaeochromocytoma, and patients on non-selective beta-blockers (risk of hypertensive crisis); controlled cardiovascular disease is NOT a contraindication — the benefit of pain control outweighs the minimal cardiovascular risk of dental doses of adrenaline
Absolute contraindications for adrenaline-containing LA are rare: unstable angina, recent MI (<6 months), uncontrolled cardiac arrhythmias, uncontrolled hyperthyroidism, and phaeochromocytoma. Relative contraindications: patients on non-selective beta-blockers (propranolol) — adrenaline causes unopposed alpha-receptor stimulation leading to hypertension and reflex bradycardia; tricyclic antidepressants (enhanced pressor response). Well-controlled hypertension, stable angina, and treated cardiac disease are NOT contraindications — adequate pain control with adrenaline-containing LA is safer than poor anaesthesia causing endogenous adrenaline release from pain and anxiety.
Question 6: What is the difference between bactericidal and bacteriostatic antibiotics, and give examples of each used in dental practice?
- There is no clinical difference between bactericidal and bacteriostatic antibiotics
- Bactericidal antibiotics kill bacteria directly (e.g., amoxicillin, metronidazole); bacteriostatic antibiotics inhibit bacterial growth and division, relying on the host immune system to eliminate the organisms (e.g., tetracycline, erythromycin/clarithromycin); in immunocompromised patients, bactericidal agents are preferred (Correct answer)
- Bacteriostatic antibiotics are always more effective than bactericidal ones
- All antibiotics used in dentistry are bacteriostatic
Correct answer: Bactericidal antibiotics kill bacteria directly (e.g., amoxicillin, metronidazole); bacteriostatic antibiotics inhibit bacterial growth and division, relying on the host immune system to eliminate the organisms (e.g., tetracycline, erythromycin/clarithromycin); in immunocompromised patients, bactericidal agents are preferred
Bactericidal antibiotics cause bacterial death: penicillins (amoxicillin) disrupt cell wall synthesis causing osmotic lysis; metronidazole forms toxic metabolites that damage bacterial DNA; cephalosporins also inhibit cell wall synthesis. Bacteriostatic antibiotics inhibit growth/replication without directly killing: tetracyclines inhibit 30S ribosomal protein synthesis; macrolides (erythromycin, clarithromycin) inhibit 50S ribosomal protein synthesis. The distinction matters most in immunocompromised patients or severe infections where the host's immune system may be insufficient to clear inhibited but viable bacteria. Most dental infections in healthy patients respond to either type.
MJDF Pharmacology and Therapeutics
What is the maximum recommended dose of lidocaine with 1:80,000 adrenaline for a healthy adult weighing 70 kg, and how many 2.2 mL cartridges does this equate to?