Common Drug interactions

Question 1

warfarin with naproxen

Answer 1

Naproxen, and other non-steroidal anti-inflammatory drugs (NSAIDs), can prolong bleeding times and cause gastrointestinal toxicity. These effects are aggravated when given with anticoagulants (e.g. warfarin)

naproxen can displace warfarin from the plasma protein binding sites, leading to more unbound warfarin available, increasing its pharmacological effect (as well as toxic effects):5,6

Question 2

phenytoin with desogestrel

Answer 2

Phenytoin is a potent CYP3A4 inducer which induces the metabolism of oral progesterone-only contraceptives (e.g. desogestrel), subsequently reducing their effect and potentially allowing ovulation to occur

Question 3

clarithromycin with simvastatin

Answer 3

Clarithromycin induces the CYP3A4 enzyme responsible for metabolising simvastatin, subsequently increasing the plasma concentrations of simvastatin.This interaction can potentially cause toxicity, leading to myopathies and rhabdomyolysis.8

Question 4

methotrexate with NSAIDs

Answer 4

Furthermore, methotrexate is cleared unchanged from the body by renal excretion. NSAIDs can increase methotrexate concentrations due to their nephrotoxic effects. There is evidence that methotrexate clearance reduces by approximately 40% following ibuprofen use, potentially causing accumulation of methotrexate and toxic adverse effects.

Question 5

synergistic interaction: ramipril with amlodipine

Answer 5

Ramipril and amlodipine work synergistically to reduce blood pressure at an enhanced level due to the different mechanisms of action:Action: no action is required unless hypotension occurs.

Question 6

harmful additive/synergistic interaction: enoxaparin with apixaban

Answer 6

The additive effects of concurrent administration of enoxaparin with apixaban cause additive anti-Xa activity. This increases the risk of bleeding.

Question 7

antagonism interaction: propranolol with salbutamol

Answer 7

Non-cardioselective beta-blockers (such as propranolol) block beta-2 receptors in the bronchi, reducing normal bronchodilation, which can exacerbate bronchoconstriction in patients with asthma.8 Similarly, non-cardioselective beta-blockers may antagonise the bronchodilator effects of beta-agonists (such as salbutamol), potentially causing bronchospasms in patients with respiratory conditions.

Question 8

Omeprazole with clopidogrel

Answer 8

Omeprazole can decrease the antiplatelet effects of clopidogrel
Consider switching to a different PPI (except esomeprazole) or an H2-receptor antagonist (except for cimetidine)

Question 9

SSRIs with NSAIDs

Answer 9

SSRIs can increase the risk of upper gastrointestinal bleeding when given with NSAIDsAdvise patients on the risk of bleeding, and consider using alternative analgesia

Question 10

Methotrexate with trimethoprim

Answer 10

Risk of severe bone marrow suppression & subsequent pancytopenia (may be fatal) when given concurrently
Monitor full blood count when administered concurrently.
Consider using folinic acid as an antidote if needed

Question 11

Verapamil with beta-blockers

Answer 11

Additive cardiac depression effects (leading to bradycardia, asystole, sinus arrest)

This interaction can also occur with ocular beta blockers
ntravenous verapamil should not be given. Oral verapamil can have some benefits but must be closely monitored

Question 12

ACE inhibitors with potassium-sparing diuretics (e.g. spironolactone/eplerenone)

Answer 12

Concurrent use increases the risk of hyperkalaemia and acute kidney injury
Use the lowest possible doses; the maximum dose of spironolactone should not exceed 25mg

Question 13

anticholinergic side effects

Answer 13

https://westessexccg.nhs.uk/your-health/medicines-optimisation-and-pharmacy/clinical-guidelines-and-prescribing-formularies/04-central-nervous-system/61-anticholinergic-side-effects-and-prescribing-guidance/file