Taking warfarin? Using supplements without telling your doctor or pharmacist could put your health at risk

A study by SingHealth Polyclinics (within April to June 2010) found that 42 per cent of patients taking warfarin, a blood thinner, used herbal or health supplements at the same time.

Mixing and not informing

Pharmacists surveyed 100 patients treated at Bedok, Bukit Merah, Pasir Ris and Tampines Polyclinics who were on warfarin. These patients were mainly taking the blood thinner to prevent stroke and heart attack.

Out of these 100 patients, 42 took supplements such as Traditional Chinese Medicines, omega-3 fatty acids and glucosamine which could potentially interact with warfarin. “What was of concern was that close to 60 per cent of these patients who were taking supplements did not inform their healthcare professionals and many of them were elderly patients who may be more sensitive to the effects of warfarin,” says lead author Bandy Qiuling Goh, Clinic Pharmacy Manager, from the Department of Pharmacy, SingHealth Polyclinics (SHP), a member of the SingHealth​ group.

Why patients failed to inform their healthcare providers despite knowing that they should

  • Since they felt well after ta​king the supplements, they perceived the supplements to be safe for consumption.
  • They thought it was not important because their healthcare providers didn’t ask them about it.
  • They were afraid that the healthcare provider might ask them to stop taking their supplements.
  • They assumed that healthcare providers were not concerned with the use of ‘non-prescription’ supplements.

More harm than good

In fact, warfarin and supplements can interact in many ways. The interactions may increase the risk of bleeding with warfarin or cause warfarin to be ineffective.Below is a list of herbal/health supplements that can interact with warfarin. This list is not exhaustive.


Coleus Ginseng (Panax)  Prickly ash 


CordycepsGinseng (Siberian)Red clover
BladderwrackDan Shen (Salvia Species)GrapeseedRhubarb
BromelainDong QuaiGreen teaSaw palmetto
CapsaicinEvening primrose oilGuggulSt John’s wort
Cat’s ClawFenugreekHorse chestnut seedSweet clover
CeleryFish oils (Omega-3 acids)Horseradish


ChamomileGarlicKavaVitamin A
CinchonaGingerLicoriceVitamin E
Clove oilGingko bilobaMelatoninVitamin K
Coenzyme Q-10Ginseng (Amer​ican)PapainWhite willow

Source: “MOH Clinical Pharmacy Practice Guidelines: Anticoagulation – Warfarin (2006)” & “Drug Information Handbook (International) 18th edition (2009)”.

Patients on warfarin may not be aware of the potential risk of interaction with supplements because they perceive the use of supplements to be innocuous. The study authors advised patients taking warfarin to consult their healthcare provider before taking any supplements, and for the healthcare provider to always educate patients who are on warfarin about the dangers of self-medication with over-the-counter herbs and supplements.

Ref: T12