The latest dispatch from the frontiers of science is that when it comes to treating your body right, sex really matters. We're talking about X and Y chromosomes, not something X-rated. See, thanks to hormones and a slew of other factors, biological differences mean your brain, nerves, heart, skin . . . every cell in your body . . . works a little differently, depending on whether you're a she or a he.

So when it comes to health, docs can't expect to treat everyone the same and get perfect results every time. As researchers enroll more women in medical studies and analyze the results by gender, important and unexpected differences are turning up all the time. (And they go far beyond who sees the dirty dishes in the sink and who can walk right by.)

These biologic gender differences are overturning some long-held beliefs about good medicine and are beginning to help doctors tailor strategies to you. Two recent examples: A daily low-dose aspirin may be great for guys' tickers at any age, but it won't protect most women from heart attacks before age 65. Yet it could lower stroke risk for women over age 55. If you're a woman under age 65 and are taking low-dose aspirin to prevent heart disease, ask your doctor if you really need it. And a study published in the Archives of Internal Medicine found that women with advanced heart failure didn't live longer with implantable cardioverter-defibrillators (a device that jump-starts a stalled heart), though men did.

Those are far from the only findings in gender-based medicine. The following five areas are the tip of an iceberg that will increasingly be unveiled:

• Heart health: Women are more likely than men to have a thin layer of dangerous, hard-to-detect plaque lining the walls of arteries in the heart, according to a recent National Institutes of Health study of 1,000 women. Called coronary microvascular syndrome, it raises heart attack risk but can't be spotted on regular heart scans called angiograms. Women with persistent chest pain plus other heart-disease risk factors may have it.


• Pain: Women seem to handle pain better than men (childbirth, anyone?). The female and male brains are thought to be wired differently when it comes to sensing pain, which is one reason some pain drugs, called kappa opiates, actually work better in women. But no one's found a fix yet for the fact that women have more migraines, arthritis, and other forms of chronic pain than guys do.


• Drug reactions: Hormones, smaller bodies and organs, and higher levels of body fat -- but not enough size- or gender-tailored drug doses -- may explain why women have 50% more adverse drug reactions than men. The sexes also seem to metabolize some drugs differently. Women are three times more likely to wake up during surgery, and they get more benefits from the blood pressure drug verapamil and the antibiotic erythromycin. The downside: Monthly hormone changes can interfere with antidepressants, and oral contraceptives can make the painkiller acetaminophen less effective. Recently, it's been speculated that women may need smaller doses of the flu vaccine, but data are not well established yet.


• Quitting smoking: Tobacco hits women's hearts and lungs even harder than men's. A pack a day increases her heart attack risk sixfold and his threefold, and it also raises women's diabetes risk and levels of "lousy" LDL cholesterol higher than it does for men. Now we need better ways to help women kick the habit: Female nicotine withdrawal symptoms may be stronger, and rumors of weight gain being greater (it isn't) make women less likely to try to quit.


• Autoimmune diseases: Women are nearly three times more likely than men to develop a disease such as lupus, multiple sclerosis, or rheumatoid arthritis, in which the immune system attacks the body. The cause? It's possible that women have stronger immune systems, that makeup and fragrances increase immune responses, that testosterone may protect men, or that there are varying levels of enzymes in each gender that dampen inflammation.