And this is exactly the sort of crazy nonsense that happens when you give medical information to laypeople without explaining it properly. Warfarin doesn't cause strokes, or if it does, it does so as a crazy and crazy unlikely side-effect in a vanishingly small number of patients. The common, dangerous, life-threatening side effect of Warfarin, the one you need to be worried about with a Warfarin sensitivity, is bleeding. Of course, Warfarin, when administed in a careful, low dosage to people with the sensitivity, may fail to prevent a stroke, but Warfarin almost certainly did not give the previous commenter's grandma a stroke, and it's driving me slightly crazy that this thread is causing people to think that it's some sort of common side effect.
What if the test is inherently flawed?
"look at what this drug did to my grandma"
Uh, that is purely anecdotal, n of 1 data, which is not scientifically valid evidence. This is what scientifically valid evidence begins to look like: http://www.nejm.org/doi/full/10.1056/NEJMoa1310669, which suggests the _opposite_ of what 23andMe says about warfarin sensitivity.
The point isn't that we shouldn't have tests for things such as warfarin sensitivity or statin sensitivity; it's simply that we need to have clinically VALIDATED tests for these medical conditions. Otherwise, without a clear understanding of the analytic and clinical validity of these tests, it's possible that many false positives or false negatives arise.
There's a reason why other genetic tests (e.g. Genomic Health's Oncotype DX or Myriad's BRCAnalysis) haven't been pulled by the FDA (in fact, they're not even regulated by the FDA!) - that's because they've gathered sufficient data to support the clinical outcomes and utility claims that they've made. Put more simply, these other molecular diagnostics have actually verified their science. 23andMe has not - that's why they're being regulated.
None of this DNA testing stuff is super-accurate.
Then you repeat the test. Your doctor may not have been considering that you may have a warfarin sensitivity, the doctor's family history may be incomplete, etc. etc. This is especially an issue with minority communities, whose medical issues are not as well dealt with by physicians, or medical science in general (minority = fundamentally less statistical strength). It's not a well-known fact, for example that asians tend to have statin sensitivity because of impaired clearance, a lower dose is equally effective in asians (although IIRC while effective in reducing cholesterol, endpoint efficacy of statins in preventing coronary disease is unknown in asians); to my horror I found my father was being prescribed the 'caucasian' dosage of statins, and then a year later, I took the 23andMe test and found that I'm homozygous to a codominant allele that is associated with dramatic increase the risk of myopathy when taking simvastatin. So here my dad is taking an inappropriate dosage of a drug that is likely making him sicker and maybe not even helping prevent coronary disease.
Now, the question you have to ask yourself, is, is it worth it to deny responsible people like myself the power to easily and cheaply obtain these sorts of decisions just to protect people who are going to get hysterical about information, even when there's a second layer of professional expertise that intervenes before any medical action is taken?
look at what this drug did to my grandma
But that is exactly, scientifically valid evidence. I think you are placing an unreasonable reverence on the 'validity' of medical 'facts' or the competence of medical authorities. After all, proper scientists aren't necessarily seeking provable facts but rather, disprovable assertions.