MJDF Pharmacology and Therapeutics

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?

Accepted Answer

7 cartridges (approximately 308 mg lidocaine) — based on the maximum dose of 4.4 mg/kg for lidocaine with adrenaline

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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.

MJDF Part 1 Practice Test

MJDF Part 1 Practice Test

MJDF Pharmacology and Therapeutics