Two drugs. One property apart.
18 side-by-side comparisons of the pairs exams confuse deliberately. Each one names the single property that separates them, then follows it through to every clinical difference it causes.
Why comparison questions are worth revising separately
Exams rarely ask what a drug does; they describe a patient and ask which of two similar drugs you would choose. That format is harder than recall because it cannot be answered from a memorised list. It requires knowing which property actually differs and whether that property matters for the patient described, which is a different skill and needs its own practice.
The good news is that almost every pair reduces to one property. Selectivity separates propranolol from atenolol. Chain length separates heparin from enoxaparin. Position in the nephron separates a loop diuretic from a thiazide. Once you hold that property, the rest of the comparison is derivable rather than memorised.
Cardiovascular
Heparin vs Warfarin
Heparin acts on clotting factors that already exist, so it works immediately, is monitored by aPTT and is reversed with protamine.
Full comparison →CompareWarfarin vs DOACs
Warfarin blocks the synthesis of several clotting factors at once, which is why it needs INR monitoring; a DOAC binds one activated factor directly, which makes its effect predictable enough to give at a fixed dose.
Full comparison →CompareUnfractionated heparin vs Enoxaparin
Enoxaparin is heparin cut into shorter chains, and chain length is the whole comparison.
Full comparison →ComparePropranolol vs Atenolol
Propranolol is non-selective and lipophilic; atenolol is beta-1 selective and hydrophilic.
Full comparison →CompareMetoprolol vs Atenolol
Both are beta-1 selective, so selectivity does not separate them.
Full comparison →CompareAmlodipine vs Nifedipine
Both are dihydropyridine calcium channel blockers acting on vascular smooth muscle, and the difference is entirely how fast they act.
Full comparison →CompareFurosemide vs Bendroflumethiazide
Furosemide blocks the sodium-potassium-two-chloride transporter in the thick ascending limb, which handles a large share of filtered sodium, making it potent.
Full comparison →CompareSpironolactone vs Eplerenone
Both block the mineralocorticoid receptor, but spironolactone is a steroid that also binds androgen and progesterone receptors, which is what causes gynaecomastia and menstrual irregularity.
Full comparison →CompareSimvastatin vs Atorvastatin
Atorvastatin is higher intensity and has a long half-life, so it can be taken at any time of day.
Full comparison →Analgesia and inflammation
Ibuprofen vs Naproxen
Both are non-selective COX inhibitors, and the difference is duration: ibuprofen has a half-life of about two hours and is taken three or four times daily, while naproxen lasts around fourteen hours and is taken twice daily.
Full comparison →CompareParacetamol vs Ibuprofen
Ibuprofen inhibits COX peripherally, so it is anti-inflammatory and carries gastric and renal risk.
Full comparison →Respiratory and allergy
Central nervous system
Sertraline vs Fluoxetine
Fluoxetine has an unusually long half-life with an active metabolite lasting days, so it rarely causes discontinuation symptoms but needs a long washout before an MAOI.
Full comparison →CompareDiazepam vs Lorazepam
Diazepam is metabolised by the liver into long-lasting active metabolites, so its effect accumulates and persists.
Full comparison →Endocrine
Prednisolone vs Dexamethasone
Dexamethasone is roughly six to seven times more potent as a glucocorticoid than prednisolone and has essentially no mineralocorticoid activity, so it does not cause the sodium and water retention prednisolone can.
Full comparison →CompareMetformin vs Sulfonylureas
Sulfonylureas close the ATP-sensitive potassium channel on the beta cell, releasing insulin whether or not glucose is high, which is why they cause hypoglycaemia and weight gain.
Full comparison →Start from the class, not the pair
A comparison only makes sense once you hold the shared mechanism, because what separates two drugs is always a variation on something they have in common. If a pair is not clicking, read the drug class first and come back, then test the distinction with the practice questions. Where the difference turns on a number rather than a mechanism, the free calculators show the working.
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Common questions
How should I revise confusable drug pairs?
Find the single property that separates them, then derive the rest from it rather than learning two parallel lists. Heparin and warfarin differ because one acts on clotting factors that already exist and the other stops new ones being made, and every difference in onset, monitoring and reversal follows from that.
Why do exams use X versus Y questions so heavily?
Because recognising a drug is a weak test and choosing between two similar drugs is a strong one. A comparison question checks whether you understand the property that matters clinically, which is much harder to answer from memorised facts alone.
Are drugs in the same class interchangeable?
Often not, and that is exactly what these pages test. Metoprolol succinate has heart failure mortality evidence and atenolol does not, despite both being cardioselective beta blockers, so class effects cannot be assumed.
Study aid only. These pages are written for learning and examination practice. They are not medical advice, not clinical decision support, and must never be used to make a decision about a real patient. Always verify against your local formulary, the product literature and a qualified pharmacist. See our medical disclaimer.
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