Primary Research · 1999
Potentiation of warfarin by dong quai
Page et al. · Pharmacotherapy, 1999
Key finding
INR rose from therapeutic range to 4.05, then 4.90 (both greater than 2x baseline). No bleeding events. INR returned to acceptable levels 1 month after discontinuing dong quai. No other changes in diet, medication, or health status were identified.
Abstract
PubMed · PMID 10417036 →Dong quai is a Chinese herbal supplement touted for treatment of menstrual cramping, irregular menses, and menopausal symptoms. Phytochemical analyses found it to consist of natural coumarin derivatives, as well as constituents possessing antithrombotic, antiarrhythmic, phototoxic, and carcinogenic effects. A 46-year-old African-American woman with atrial fibrillation stabilized on warfarin experienced a greater than 2-fold elevation in prothrombin time and international normalized ratio after taking dong quai concurrently for 4 weeks. No identifiable cause was ascertained for the increase except dong quai. The patient's coagulation values returned to acceptable levels 1 month after discontinuing the herb. One animal study suggests a pharmacodynamic interaction between the product and warfarin, but the true mechanism remains unknown. Practitioners should be aware of the possibility of such an interaction and should inform patients of potential hazards of taking the two together.
Abstract sourced from PubMed, a database of the U.S. National Library of Medicine. Displayed in the authors’ own words for context; our critique is in the sections below.
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How to read a study like this
The same questions worth asking about any research paper, not just this one. Worth a minute even if you trust the grade.
Who was studied, and do you resemble them?
Supplement effects often depend on baseline status. Vitamin D helps people who are deficient; iron helps people who are anemic. A result in people unlike you may not apply to you.
What was measured, and does it matter in daily life?
A study that shows a blood marker moved isn't the same as a study that shows people felt or functioned better. Ask what the outcome means in practice.
How large was the effect — not just whether it was significant.
'Statistically significant' only means the effect is unlikely to be zero. It doesn't tell you the effect is large enough to notice. Look for effect sizes, not just p-values.
Who paid for the trial, and what did they stand to gain?
Industry-funded trials are several times more likely to report positive results than independent ones. It's not usually fraud — it's subtle design and reporting choices. Weight accordingly.
Has anyone else replicated this?
Single positive trials are hypotheses. Replication by independent groups is what turns a hypothesis into reliable evidence. If the only positive trial is the one you're reading, wait.
Does the dose in the trial match what's being sold?
Supplement marketing routinely cites trials that used 5–10× the dose in the product. If the effective dose was 2 g/day and the capsule has 200 mg, expect roughly no effect.
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