A woman develops a urinary-tract infection, and her doctor prescribes a standard treatment, Bactrim. But the doctor forgets to ask about her ethnic heritage, and she's a Sephardic Jew. In up to 30 percent of that group of people, originally from North Africa, Bactrim and other sulfa drugs can destroy red blood cells, a potentially fatal side effect.

Such oversights are all too common."While every good doctor wants to tailor treatment to the individual patient, that's not as easy as you might think," says Jay Cohen, M.D., who has advocated extensively for an individualized approach to health care. That's partly because studies have focused largely on healthy, white, middle-aged men. And their findings usually represent group averages, which may not apply to specific individuals. As result, "the generally recommended therapy is often not the best choice for any given person," Cohen says.

Instead, determining the optimal treatment for any condition requires you and your doctor to consider many factors, such as your age, gender, race, physique, other health problems, and personal preferences. But it's only natural for busy doctors, particularly when harried by time pressures, to slip into treating you like just one of the crowd.

The table shows important exceptions to common advice on nonmedical treatments. Below, we describe the nine most important factors you and your doctor should consider when tailoring tests and treatments specifically to you:

Age: Consider the changes
Gender: Beware the unisex trap
Family history: Tell the tales
Ethnic heritage: Race matters
Health problems: Factor them in
Your preferences: Heed your wishes
Drugs, supplements: Watch for interactions
Unusual causes: Think zebras
Physique: Weighty distinctions

This article first appeared in the June 2007 issue of Consumer Reports on Health.