What this means in practice
Nothing changes for most people. Anyone taking warfarin already has their International Normalized Ratio checked on a schedule, and that check would pick up a real effect long before a supplement label could predict one. Telling the clinician who manages the dose about any supplement is standing advice and is not specific to this one.
What people say
Lion's mane thins your blood. Do not take it with warfarin or any blood thinner, and stop it two weeks before surgery.
What the evidence supports
One screen tested hot-water extracts of eight edible mushrooms, the preparation closest to how a mushroom is actually eaten, on human platelets and on clotting times in the laboratory. Lion's mane was not among the species that slowed platelet clumping; jelly ear, king oyster and reishi were. None of the eight changed prothrombin time, prothrombin ratio or the International Normalized Ratio, which are the measurements a warfarin dose is set by. No study has given lion's mane to a person taking warfarin.
How it is supposed to work
Some mushroom extracts interfere with the signals platelets use to clump together. An earlier laboratory report described a lion's mane extract blocking one of those signals at a platelet receptor, and that is where the bleeding worry comes from.
Which way it runs
The claim runs one way only: lion's mane is said to add to a blood thinner's effect, not the reverse. Nothing in the sourced evidence suggests warfarin changes what lion's mane does.
What is still missing
The caution is stated as a fact about the body and rests on a tube. Nobody has given lion's mane to a person on warfarin, and the one screen that went looking for the effect in lion's mane did not find it.