Vick's First Defence works by lining the nose to produce a barrier that cold viruses can't cross. So I wouldn't have thought there would be any interaction with warfarin. I have used it before but I'm not sure it helped much and I didn't notice any impact on my INR.
I use First Defence without any problems and think it very effective. Avoid direct contact with Brother in Law and wash hands thoroughly as soon as you get there.
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