CarolC makes the suggestion of using the result of the drug's use to determine whether the patient is black or not.
This is a beautiful example of the sort of logical fallacy that bedevils 90% of discussions about race, and also about medical things like "what causes cancer".
The clearest example (often quoted) of this fallacy is
1) People with measles tend to have a high temperature.
1) Therefore, people with high temperatures have probably got measles.
Unfortunately,(2) does not follow from (1), though a huge proportion of the arguing classes seem to think it does.