MJDF Pharmacology and Therapeutics 2 — Questions and Answers
Question 1: What is the pharmacological basis for combining paracetamol and ibuprofen for post-operative dental pain?
- They should never be combined as they interact dangerously
- They have different mechanisms of action (paracetamol: central COX inhibition and possible serotoninergic pathways; ibuprofen: peripheral COX-1 and COX-2 inhibition) providing additive or synergistic analgesia with fewer side effects than increasing the dose of either drug alone (Correct answer)
- They have identical mechanisms and combining them offers no advantage
- Combining them doubles the risk of liver damage
Correct answer: They have different mechanisms of action (paracetamol: central COX inhibition and possible serotoninergic pathways; ibuprofen: peripheral COX-1 and COX-2 inhibition) providing additive or synergistic analgesia with fewer side effects than increasing the dose of either drug alone
Paracetamol and ibuprofen act through different mechanisms: paracetamol primarily acts centrally (inhibiting COX in the CNS, possibly involving the endocannabinoid system and serotoninergic pathways), while ibuprofen acts peripherally (inhibiting COX-1 and COX-2 at the site of tissue damage, reducing prostaglandin-mediated inflammation and pain). Combining them provides additive analgesia — studies show paracetamol 1000 mg + ibuprofen 400 mg is more effective than either alone and comparable to some weak opioid combinations, without the opioid side effects. They can be taken simultaneously or alternated, within recommended dose limits for each.
Question 2: A patient taking the oral contraceptive pill is prescribed a course of antibiotics for a dental infection. What is the current guidance regarding potential interaction?
- All antibiotics reduce the effectiveness of the oral contraceptive pill and additional contraception is always needed
- Current UK guidance (FSRH and BNF) states that non-enzyme-inducing antibiotics (amoxicillin, metronidazole, clarithromycin) do NOT reduce the efficacy of combined hormonal contraception; additional contraception is NOT required. Only enzyme-inducing drugs (rifampicin, rifabutin) significantly reduce contraceptive efficacy (Correct answer)
- Antibiotics enhance the effectiveness of the contraceptive pill
- This interaction has never been studied
Correct answer: Current UK guidance (FSRH and BNF) states that non-enzyme-inducing antibiotics (amoxicillin, metronidazole, clarithromycin) do NOT reduce the efficacy of combined hormonal contraception; additional contraception is NOT required. Only enzyme-inducing drugs (rifampicin, rifabutin) significantly reduce contraceptive efficacy
Previous advice to use additional contraception with broad-spectrum antibiotics has been superseded. The Faculty of Sexual and Reproductive Healthcare (FSRH) and the BNF now state that non-enzyme-inducing antibiotics (including amoxicillin, metronidazole, erythromycin, clarithromycin, tetracyclines) do NOT reduce the efficacy of combined hormonal contraceptives. The earlier concern about disruption of enterohepatic recirculation of ethinylestradiol has not been supported by pharmacokinetic studies. Only enzyme-inducing drugs (rifampicin, rifabutin, some anticonvulsants, some antiretrovirals) significantly affect contraceptive efficacy. Additional contraception is only needed with these specific drugs.
Question 3: What are the signs and symptoms of local anaesthetic toxicity and how should it be managed in the dental surgery?
- LA toxicity only causes numbness and has no systemic effects
- Early signs include perioral tingling, metallic taste, tinnitus, light-headedness, and visual disturbance, progressing to tremors, seizures, respiratory depression, and cardiovascular collapse; management includes stopping injection, positioning supine, maintaining airway, administering oxygen, treating seizures with IV midazolam, and calling 999 — IV lipid emulsion (Intralipid 20%) is the specific antidote for severe cardiovascular toxicity (Correct answer)
- LA toxicity is always fatal and cannot be treated
- It presents as an allergic rash only
Correct answer: Early signs include perioral tingling, metallic taste, tinnitus, light-headedness, and visual disturbance, progressing to tremors, seizures, respiratory depression, and cardiovascular collapse; management includes stopping injection, positioning supine, maintaining airway, administering oxygen, treating seizures with IV midazolam, and calling 999 — IV lipid emulsion (Intralipid 20%) is the specific antidote for severe cardiovascular toxicity
LA toxicity occurs from excessive plasma levels (overdose, inadvertent IV injection, or impaired metabolism). The progression follows CNS then CVS depression: CNS — perioral tingling, metallic taste, tinnitus, visual disturbance, light-headedness → slurred speech, drowsiness → muscle twitching → generalised seizures → unconsciousness. CVS — hypotension → bradycardia → arrhythmias → cardiac arrest. Management: stop administration, call for help, maintain airway (head tilt-chin lift), high-flow oxygen, treat seizures (midazolam 0.1-0.2 mg/kg IV or buccal), IV fluids, call 999. For cardiac arrest refractory to standard ALS: IV lipid emulsion (Intralipid 20%) 1.5 mL/kg bolus followed by infusion — this acts as a 'lipid sink' extracting lipophilic LA from cardiac tissue.
Question 4: What is the mechanism of action of metronidazole and why is alcohol consumption contraindicated during its use?
- Metronidazole inhibits cell wall synthesis; alcohol is contraindicated because it reduces absorption
- Metronidazole enters anaerobic bacteria where it is reduced to form cytotoxic intermediates that damage bacterial DNA; alcohol is contraindicated because metronidazole inhibits aldehyde dehydrogenase, causing accumulation of acetaldehyde leading to a disulfiram-like reaction (nausea, vomiting, flushing, headache, tachycardia) (Correct answer)
- Metronidazole inhibits protein synthesis; alcohol enhances its antibacterial effect too much
- Metronidazole has no interaction with alcohol — this is a myth
Correct answer: Metronidazole enters anaerobic bacteria where it is reduced to form cytotoxic intermediates that damage bacterial DNA; alcohol is contraindicated because metronidazole inhibits aldehyde dehydrogenase, causing accumulation of acetaldehyde leading to a disulfiram-like reaction (nausea, vomiting, flushing, headache, tachycardia)
Metronidazole is a nitroimidazole antibiotic that is selectively toxic to anaerobic organisms. Inside anaerobic bacteria, the nitro group is reduced by ferredoxin (electron transport protein) to form cytotoxic intermediates (nitroso free radicals) that damage DNA strands, causing cell death. It is bactericidal against obligate anaerobes but has no activity against aerobes. The alcohol interaction occurs because metronidazole inhibits aldehyde dehydrogenase, the enzyme that metabolises acetaldehyde (a toxic metabolite of alcohol). Accumulation of acetaldehyde causes a disulfiram-like reaction with nausea, vomiting, flushing, abdominal cramps, and headache. Patients should avoid alcohol during treatment and for 48 hours after completion.
Question 5: What are the key considerations when prescribing analgesics for a pregnant patient experiencing dental pain?
- All analgesics are safe during pregnancy
- Paracetamol is the first-line analgesic throughout pregnancy; NSAIDs (ibuprofen) should be avoided after 28 weeks (risk of premature closure of the ductus arteriosus) and used with caution before this; codeine should be used at the lowest effective dose for the shortest duration; aspirin is contraindicated in the third trimester (Correct answer)
- No analgesics can be given during pregnancy under any circumstances
- Opioid analgesics are the first-line choice in pregnancy
Correct answer: Paracetamol is the first-line analgesic throughout pregnancy; NSAIDs (ibuprofen) should be avoided after 28 weeks (risk of premature closure of the ductus arteriosus) and used with caution before this; codeine should be used at the lowest effective dose for the shortest duration; aspirin is contraindicated in the third trimester
Prescribing in pregnancy follows the principle of minimum effective dose for the shortest time. Paracetamol (all trimesters) is first-line — extensive safety data, no teratogenic effects at therapeutic doses. Ibuprofen: relatively safe in the first and second trimesters but CONTRAINDICATED after 28 weeks as it may cause premature closure of the fetal ductus arteriosus, reduced fetal renal function, and oligohydramnios. Codeine: use cautiously — maternal use near delivery can cause neonatal respiratory depression and withdrawal; ultrarapid CYP2D6 metabolisers convert codeine to morphine rapidly. Aspirin: avoid in third trimester (bleeding risk, ductus arteriosus closure). Always consider whether treatment can be deferred until after delivery.
Question 6: What is antimicrobial resistance and how can dental practitioners contribute to antimicrobial stewardship?
- Antimicrobial resistance is not relevant to dentistry
- Antimicrobial resistance occurs when bacteria evolve mechanisms to survive antibiotic exposure; dental practitioners contribute to stewardship by prescribing antibiotics only when clinically indicated, choosing narrow-spectrum agents, using appropriate doses and durations, prioritising drainage/removal of the cause over antibiotics alone, and following evidence-based prescribing guidelines (FGDP/SDCEP) (Correct answer)
- Dental practitioners should prescribe antibiotics for every patient to prevent resistance
- Antimicrobial resistance can be prevented by using antibiotics prophylactically for all procedures
Correct answer: Antimicrobial resistance occurs when bacteria evolve mechanisms to survive antibiotic exposure; dental practitioners contribute to stewardship by prescribing antibiotics only when clinically indicated, choosing narrow-spectrum agents, using appropriate doses and durations, prioritising drainage/removal of the cause over antibiotics alone, and following evidence-based prescribing guidelines (FGDP/SDCEP)
Antimicrobial resistance (AMR) is a global health threat. Bacteria develop resistance through genetic mutation or horizontal gene transfer, producing enzymes that degrade antibiotics (beta-lactamases), altering drug targets, increasing efflux pumps, or reducing permeability. Dental prescribing accounts for approximately 5-10% of all antibiotic prescriptions in England. Stewardship measures include: only prescribing when clinically indicated (not for irreversible pulpitis, dry socket, or mild pericoronitis), addressing the cause (drainage, extraction) rather than relying on antibiotics, using narrow-spectrum first-line agents, following FGDP/SDCEP guidelines, using the correct dose and duration, and educating patients that antibiotics are not a substitute for dental treatment.
MJDF Pharmacology and Therapeutics 2
What is the pharmacological basis for combining paracetamol and ibuprofen for post-operative dental pain?