Pharmacology and Therapeutics
What this domain tests
Items test local anaesthetic selection and maximum doses, antibiotic choice and prophylaxis rules, analgesic and anxiolytic prescribing, and clinically important drug interactions with dental treatment.
Local anaesthetics
- Lidocaine 2% with 1:100,000 adrenaline: 20 mg/mL, so a 1.8 mL cartridge holds 36 mg. Maximum 4.4 mg/kg (7 mg/kg with vasoconstrictor) and an absolute cap of about 300-500 mg.
- Articaine 4% is metabolised by plasma esterases and penetrates bone well; 4% formulations are linked to more reports of persistent paraesthesia after mandibular blocks.
- Bupivacaine 0.5% with adrenaline is long-acting; prilocaine can cause methaemoglobinaemia; mepivacaine 3% plain is used when vasoconstrictor is unwanted.
- Adrenaline limits: 0.2 mg in a healthy adult (about 11 cartridges of 1:100,000), 0.04 mg in significant cardiac disease (about 2 cartridges).
Antimicrobials and prophylaxis
- Amoxicillin 500 mg three times daily is first line for odontogenic infection; penicillin V, clindamycin and metronidazole for anaerobes are alternatives.
- Antibiotics are adjuncts to drainage and removal of the cause, not substitutes.
- Infective endocarditis prophylaxis (2 g amoxicillin 30-60 minutes preoperatively) is reserved for highest-risk patients: prosthetic valve or material, previous endocarditis, and certain cyanotic congenital heart disease.
- Routine prophylaxis is no longer indicated for prosthetic joints.
Interactions and emergencies
- Metronidazole, azole antifungals and clarithromycin raise INR with warfarin.
- Adrenaline in local anaesthetic interacts with non-selective beta-blockers, tricyclics and cocaine.
- Midazolam is reversed by flumazenil; opioids by naloxone; hypoglycaemia is treated with oral glucose or IV dextrose, not more sedation.
Worked example
A 70 kg adult needs a mandibular block with 2% lidocaine and 1:100,000 adrenaline. Reasoning: 4.4 mg/kg × 70 kg = 308 mg. Each 1.8 mL cartridge holds 36 mg, so 308/36 = 8.5 cartridges by the anaesthetic cap. Adrenaline 0.2 mg per cartridge is 0.018 mg, so the 0.2 mg limit allows about 11 cartridges. The anaesthetic limit binds first in a healthy patient at roughly 8 cartridges. In a cardiac patient the adrenaline limit of 0.04 mg binds first at about 2 cartridges.
Traps
Forgetting mg/mL conversion; applying the healthy adrenaline limit to cardiac patients; prescribing antibiotics without drainage; assuming joint prophylaxis is still routine.