What this interaction is about
For glucosamine a possible influence on coumarin anticoagulants is described. In Germany that mainly means phenprocoumon; warfarin is better known internationally. Cases with a shifted INR value after additional glucosamine intake have been reported.
INR is the measure by which anticoagulation is set. If it shifts, the balance between clotting and bleeding risk shifts with it, in either possible direction.
How strong the evidence is
The information comes largely from case reports and pharmacovigilance notifications rather than large controlled trials. It is therefore not automatic.
For a medicine whose dose is set individually by laboratory values, a plausible suspicion is nevertheless enough to justify clear caution. The potential harm is significant; doing without a supplement costs nothing.
What to do in practice
Do not start a glucosamine product without first speaking to the prescribing practice or your pharmacy. Bring the pack or a photo of the label so the full composition can be assessed.
Never stop a prescribed medicine on your own in order to take a supplement. If you do start after consultation, closer INR monitoring in the first weeks is the usual approach.
Other situations needing attention
Supplements are often stopped as a precaution before planned surgery. Name everything you take, including herbal products, in the pre-operative discussion.
New bleeding signs are a reason for medical contact: unusual bruising, bleeding gums, nosebleeds or small wounds that keep oozing.