Pharmacology
This domain tests dose calculation, drug selection and interactions. Almost every question resolves to a number, a maximum dose or a contraindication.
Local anaesthetics
- Amides (lidocaine, articaine, mepivacaine, prilocaine, bupivacaine) are metabolised in the liver; esters such as benzocaine are hydrolysed by plasma esterases, so ester allergy is more common and amide allergy is rare.
- Two percent lidocaine is 20 mg per mL, with a maximum of 4.4 to 7 mg per kg. Articaine 4 percent allows 7 mg per kg. Prilocaine carries a methemoglobinemia risk. Bupivacaine is long-acting and not for children.
- Epinephrine 1:100,000 is 10 micrograms per mL, so a 1.8 mL cartridge contains 0.018 mg.
Analgesics and antibiotics
- Paracetamol 1 g four times daily, maximum 4 g per day; ibuprofen 400 to 600 mg three times daily, avoided in asthma, renal impairment and the third trimester.
- Amoxicillin 500 mg three times daily, amoxicillin-clavulanate, metronidazole 400 to 500 mg three times daily, clindamycin 300 mg four times daily.
- American Heart Association 2021 guidance limits infective endocarditis prophylaxis to high-risk cardiac conditions: 2 g amoxicillin 30 to 60 minutes before the procedure.
Interactions
Metronidazole and azole antifungals raise INR on warfarin; macrolides potentiate several drugs; epinephrine with a non-selective beta-blocker can cause hypertension; monoamine oxidase inhibitors potentiate vasoconstrictors.
Worked example
A 60 kg adult needs 2 percent lidocaine with 1:100,000. Maximum lidocaine is 4.4 mg per kg, giving 264 mg, which is 13.2 mL or 7.3 cartridges of 1.8 mL. Maximum epinephrine for a healthy adult is 0.2 mg, which is 11 cartridges. The local anaesthetic, not the vasoconstrictor, is the limiting factor here.
Traps
- Counting cartridges instead of milligrams.
- Prescribing antibiotics for irreversible pulpitis.
- Ignoring the pregnancy category.
- Missing the warfarin-metronidazole interaction.