Boxed warnings on a drug's label don't carry skulls and crossbones, but they can sometimes be a death sentence in the revenue department. The Food and Drug Administration reserves the box for only the most severe warnings, which are often life-threatening side effects.

So it came as a bit of a surprise to me when the FDA slapped a black-boxed warning on Bristol-Myers Squibb (NYSE: BMY) and sanofi-aventis' (NYSE: SNY) Plavix on Friday. The availability of a genetic test to see if the patient will respond to Plavix has been noted on the label since last May, but the agency decided to highlight the warning. I guess "not working" on some people is a side effect, but it's not at the same level as Biogen Idec (Nasdaq: BIIB) and Elan's (NYSE: ELN) Tysabri causing a potentially fatal brain infection, for instance.

How much will the boxed warning hurt sales? In theory, not at all since the availability to test was already on the label. In reality, there were doctors not ordering the test; that's the reason the FDA wanted the availability highlighted. If doctors increase their usage, some additional patients screened for the genetic variation won't end up taking the drug. The genetic difference occurs in 2% to 14% of individuals depending on their racial background.

Another issue for Bristol and Sanofi is that drugs with boxed warnings have stricter advertising requirements, including a ban on reminder ads, which contain the name of the drug, but don't say what it does.

So who's poised to pick up those patients that can't benefit from Plavix? Eli Lilly's (NYSE: LLY) Effient was approved last year, but the drug hasn't gained much traction with sales of just $27 million since its launch last year. AstraZeneca (NYSE: AZN) also has a blood thinner, Brilinta, which is up for review with the FDA.

Bristol and Sanofi are trying to milk as much out of Plavix as they can before it goes off patent in a few years. This new boxed warning may slow the stream just a bit, but with a bucket as full as it is ($6 billion and counting), the new warning shouldn't be too hard to deal with.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Elan is a Motley Fool Rule Breakers choice. The Fool has a disclosure policy.