If you’ve ever left your doctor’s office feeling more like a number than a person, that’s no surprise. Much of what happens in your doctor’s office is based on averages. If you need medication, you’ll be given a drug that works for many people but might or might not work for you. Your doctor can speculate about diseases that you might get—or how a current disease will affect you or respond to treatment—but he/she doesn’t know for sure.

What’s new: In the 10 years since the Human Genome Project was completed (sequencing revealed an estimated 3 billion pairs of DNA molecules in a person’s genome), doctors have begun to tailor medical treatments to each person’s unique molecular and genetic profile. Precision medicine (also known as “personalized medicine,” a term that includes such factors as lifestyle and personal values) uses genetics to take some of the guesswork out of diagnosing, treating and preventing diseases.

To learn more about this exciting new field, Bottom Line/Health spoke with Geoffrey S. Ginsburg, MD, PhD, one of the country’s top experts in gene-based medicine.

Hasn’t genetic testing been used for a long time? We have a lot of experience in diagnosing and treating relatively straightforward genetic diseases, such as sickle-cell anemia and muscular dystrophy.

It’s only in recent years, however, that we’ve learned that diseases that were previously linked largely to environmental and lifestyle factors also have a strong genetic component. For example, there are several genes that regulate (encode) the cell receptors for nicotine. A smoker may be more likely to get lung cancer if he has an underlying genetic propensity to the disease.

In people with type 2 diabetes, between 40 and 50 genetic mutations have been identified. This means that even if you have normal blood sugar and no other signs of disease, the presence of these mutations indicates that you have a high future risk—and will want to be particularly careful about following steps to mitigate this risk, such as watching your weight and eating a nutritious diet.

Do genetic tests tell people what diseases they are going to get? Not necessarily. But the tests can indicate, in part, the magnitude of your risk for certain diseases, ranging from heart disease and epilepsy to various cancers.

Ovarian cancer is a good example. It affects only about 1.4% of American women overall, but up to 39% of those with a mutation in the gene known as BRCA1 develop this cancer. A mutation in this gene also increases risk for breast cancer in women, prostate cancer in men and pancreatic cancer in both sexes.

If genetic tests indicate a mutation in either the BRCA1 or BRCA2 gene, more frequent screening tests, beginning at an earlier age than usual or even prophylactic removal of the ovaries and breasts in some women, may be recommended.

If a person has already been diagnosed with, say, cancer, is genetic testing still advised? The tests are not only used to identify specific types of cancer but also to customize the treatments.

For example, many breast cancer patients have genes that overexpress (produce too much of) the HER2 protein. These patients do not respond well to standard chemotherapy but can be effectively treated with a drug called trastuzumab (Herceptin).

About 50% of melanoma patients have a mutation in the BRAF gene, which controls cell growth. A chemotherapy drug called vemurafenib (Zelboraf) works only in patients with this mutation.

Is genetic testing being used for other medications? Yes. Let’s say that you need medication to prevent blood clots after a heart attack. Warfarin (Coumadin) is effective but difficult to use. A dose that’s too high can cause excessive bleeding, while a too-low dose could allow clots to form. The only way to get the right dose is with trial and error—and the risks can be high. With genetic testing, your doctor is more likely to start you with the correct dose.

Statins are another example. Many people who need a statin to lower their cholesterol levels quit taking it because of muscle pain, a relatively common side effect. We have identified two genetic variants that have been linked to myopathy (muscle pain). A patient with one of these variants might be prescribed a lower dose or switched to a statin, such as pravastatin (Pravachol), that’s less likely to cause myopathy.

To see if such testing is available for a drug you’re taking, go to www.FDA.gov/drugs/scienceresearch/researchareas. Click on “Genomics,” then “Table of Pharmacogenomic Biomarkers in Drug Labels.”

Can individuals order their own genetic tests? Yes—but ask yourself what you’ll do with the information. Some internet companies offer genetic tests directly to consumers. You can order tests to assess your risk for rheumatoid arthritis, Alzheimer’s disease, prostate cancer and many other diseases. You take a saliva sample, return it to the company and get your results in several weeks. The genetic test itself is fairly accurate, but the interpretation of the results may vary. In general, a particular mutation will probably be found if you have it, but what it means to your health is still a work in progress.

The downside is that the tests don’t address such issues as your medical history and lifestyle. Then there’s the emotional side: You might learn that you’re at risk for a serious disease—one that you can’t do anything to prevent.

The tests also can be expensive, typically ranging from $99 to more than $1,000. (The self-tests usually are not covered by insurance, while certain genetic tests ordered by your doctor typically are.)

In general, it’s a good idea to ask your doctor if you would benefit from genetic testing. If your doctor isn’t sure, consider consulting a physician at an academic medical center, where you’re more likely to find experts in this area, or a genetics counselor. The National Society of Genetic Counselors, NSGC.org, can refer you to someone in your area.