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. 2016 Apr 1;39(2):32–33. doi: 10.18773/austprescr.2016.025

Warfarin, St John’s wort and INR

Angela Cattanach 1, Siboniso Sibindi 1
PMCID: PMC4917631  PMID: 27340114

In regards to the article on the management of warfarin therapy,1 the statement on page 46 ‘drugs that may increase INR – macrolide antibiotics, imidazole antifungals, sulfamethoxazole/trimethoprim, amiodarone, statins, some non-steroidal anti-inflammatory drugs and some complementary medicines such as St John’s wort’ may not be correct.

In the literature, St John’s wort decreases the INR through induction of cytochrome P450 (CYP)-mediated metabolism of warfarin and increases warfarin clearance.2-9

References

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  • 2.Jiang X, Williams KM, Liauw WS, Ammit AJ, Roufogalis BD, Duke CC, et al. Effect of St John’s wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. Br J Clin Pharmacol 2004;57:592-9. 10.1111/j.1365-2125.2003.02051.x [DOI] [PMC free article] [PubMed] [Google Scholar]
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Aust Prescr. 2016 Apr 1;39(2):32–33.

Author’s response

Gregory W Roberts 1

Gregory Roberts, one of the authors of the article, comments:

Thank you for pointing out the error in the article.1 St John’s wort induces CYP enzymes with a resultant increase in warfarin clearance and decrease in INR, not a possible increase in INR as described in the article. Decreases of 20% in AUC (area under the curve) have been noted in single warfarin dose studies,2 so while prudent INR monitoring should be undertaken, the interaction is likely to be of clinically minor importance.

References

  • 1.Tideman PA, Tirimacco R, St John A, Roberts GW. How to manage warfarin therapy. Aust Prescr 2015;38:44-8. 10.18773/austprescr.2015.016 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Jiang X, Williams KM, Liauw WS, Ammit AJ, Roufogalis BD, Duke CC, et al. Effect of St John’s wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. Br J Clin Pharmacol 2004;57:592-9. 10.1111/j.1365-2125.2003.02051.x [DOI] [PMC free article] [PubMed] [Google Scholar]

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