Pharmacology & Prescribing Safety

A 78-year-old woman on warfarin is prescribed a course of clarithromycin for a chest infection. What is the concern?

• AThe combination causes serotonin syndrome, with agitation, sweating and a fast pulse
• BClarithromycin cannot be given to anyone over 75 because of the risk of permanent hearing loss
• CClarithromycin inhibits warfarin metabolism and can raise the INR to dangerous levels✓
• DClarithromycin will make the warfarin ineffective, so the dose needs to be increased for the week

Answer: Macrolides such as clarithromycin inhibit the cytochrome enzymes that metabolise warfarin, increasing its effect; the INR should be monitored closely or an alternative antibiotic chosen.

A patient with penicillin allergy that caused anaphylaxis needs an antibiotic for cellulitis. Which is appropriate?

• AFlucloxacillin at the high dose
• BClarithromycin or doxycycline✓
• CCo-amoxiclav at the standard dose
• DAmoxicillin for a full ten days

Answer: Flucloxacillin, amoxicillin and co-amoxiclav are all penicillins and are contraindicated after penicillin anaphylaxis. NICE recommends clarithromycin, erythromycin or doxycycline as alternatives for cellulitis.

A 45-year-old man on ramipril for hypertension is started on ibuprofen for back pain and a thiazide diuretic is added. What is the risk of this combination?

• ASevere hypotension only, which settles once the patient is used to the combination
• BHyperglycaemia from the interaction between the NSAID and the thiazide, which needs a glucose check
• CHypercalcaemia from the diuretic, made worse by the ACE inhibitor reducing calcium excretion
• DAcute kidney injury — the 'triple whammy' of an ACE inhibitor, an NSAID and a diuretic✓

Answer: ACE inhibitors, NSAIDs and diuretics each reduce renal perfusion by different mechanisms; together they are a well-recognised cause of acute kidney injury, especially in older or dehydrated patients.

A pregnant woman with epilepsy asks about her medication. Which drug carries the highest risk of major congenital malformations and should be avoided in pregnancy where possible?

• AHigh-dose folic acid
• BLevetiracetam monotherapy
• CSodium valproate✓
• DLamotrigine monotherapy

Answer: Sodium valproate causes neural tube defects and neurodevelopmental problems and must not be used in girls and women of childbearing potential unless a pregnancy prevention programme is in place. Lamotrigine and levetiracetam are the preferred options, with high-dose folic acid. Since January 2024 the MHRA has also required that valproate is not started in any patient under 55, of either sex, unless two specialists independently consider that no other treatment is effective or tolerated.

A 30-year-old woman with asthma is prescribed propranolol for anxiety. Why is this a prescribing error?

• ANon-selective beta-blockers can cause life-threatening bronchospasm in asthma✓
• BPropranolol interacts with salbutamol to cause tachycardia
• CPropranolol is only licensed for heart failure
• DBeta-blockers are contraindicated in anyone under 40

Answer: Beta-blockers, especially non-selective ones like propranolol, block the beta-2 receptors that keep the airways open and can precipitate severe bronchospasm in people with asthma; the BNF lists asthma as a contraindication.

A 65-year-old man is prescribed morphine modified-release for cancer pain. What should be co-prescribed from the outset?

• AA regular antiemetic for the whole course, since opioid-induced nausea does not settle with time
• BNaloxone to be taken every morning to reverse any overnight sedation
• CA regular laxative and an as-required breakthrough dose of immediate-release morphine✓
• DA benzodiazepine at night to prevent the respiratory depression that opioids cause during sleep

Answer: Opioid-induced constipation is almost universal, so a laxative is started with the opioid; breakthrough pain is managed with immediate-release morphine at one sixth of the daily dose. Nausea usually settles within days, so antiemetics are used only if needed.

A 70-year-old woman on lithium develops diarrhoea and vomiting and becomes drowsy with a coarse tremor. What is the most likely diagnosis and the immediate step?

• AA viral gastroenteritis — treat symptomatically with oral rehydration and continue lithium as usual
• BSerotonin syndrome — give cyproheptadine and stop any serotonergic drugs
• CHypothyroidism — start levothyroxine
• DLithium toxicity — stop lithium, check the serum level and renal function, and give intravenous fluids✓

Answer: Dehydration from diarrhoea and vomiting reduces lithium clearance and precipitates toxicity, which presents with tremor, drowsiness and ataxia. Lithium has a narrow therapeutic index and levels must be measured urgently.

A 55-year-old man with type 2 diabetes and an eGFR of 25 mL/min is on metformin. What should be done?

• ADouble the metformin dose to compensate for the reduced absorption that occurs in kidney disease
• BSwitch to a sulfonylurea at the maximum dose
• CContinue metformin at the same dose with monthly monitoring of renal function and lactate levels
• DStop metformin, because of the risk of lactic acidosis at this level of renal function✓

Answer: Metformin is reviewed when the eGFR falls below 45 and stopped below 30, because accumulation causes lactic acidosis. An alternative agent is chosen with renal function in mind.