Key points
- Vitamin K and warfarin: never adjust without INR monitoring.
- Potassium plus ACE inhibitor can cause dangerous hyperkalaemia.
- St. John’s Wort is an enzyme inducer that undermines many drugs.
Vitamin K and warfarin
Warfarin blocks vitamin K epoxide reductase. Vitamin K1 and K2 restore substrate for that enzyme and directly antagonise the drug. The result can be an INR in the clot-risk range with no warning symptom.
The guidance is consistency, not avoidance. Keep vitamin K intake steady week to week and never add a K supplement without prescriber knowledge and INR monitoring.
Potassium and blood pressure medication
ACE inhibitors reduce aldosterone-driven potassium excretion, and potassium-sparing diuretics retain potassium directly. Adding a potassium supplement on either can push serum potassium into the hyperkalaemic range, which risks cardiac arrhythmia.
Do not supplement potassium on these drugs without prescriber approval and periodic potassium and creatinine checks. Potassium from food is generally fine because the amounts are lower and self-limiting.
Minerals and levothyroxine
Calcium, iron and magnesium all bind levothyroxine in the intestinal lumen, reducing absorption by up to a third. Because levothyroxine has a narrow therapeutic index, this shows up as an unstable TSH.
Take levothyroxine first thing on an empty stomach with plain water, and keep all three minerals at least four hours away.
Minerals and antibiotics
Calcium, magnesium, iron and zinc chelate tetracycline and fluoroquinolone antibiotics into non-absorbable complexes. Absorption of the antibiotic can fall by more than half, which risks treatment failure and promotes resistance.
Take the antibiotic at least two hours before or four to six hours after any of these minerals.
St. John’s Wort: the enzyme inducer
St. John's Wort induces CYP3A4, CYP2C9 and P-glycoprotein. That accelerates clearance of warfarin, direct oral anticoagulants, many antidepressants, oral contraceptives and immunosuppressants, often to the point of therapeutic failure.
It also adds serotonergic activity, so combined with an SSRI it risks serotonin syndrome. Treat it as a medication with a long interaction list, not as a benign herb.
Grapefruit and statins
Grapefruit furanocoumarins irreversibly inhibit intestinal CYP3A4, sharply raising blood levels of simvastatin and atorvastatin and increasing rhabdomyolysis risk. The inhibition lasts until new enzyme is synthesised, so separating the two in time does not help.
Rosuvastatin and pravastatin are not CYP3A4 substrates to a meaningful degree and are not affected in the same way.