He explains that the proper dosing can range from 1 milligram to 15 milligrams, and too much of the drug can lead to serious bleeding -- bleeding that sends 30,000 people to the emergency room each year.
As for how the dose is determined, "it's all chasing your tail now," Nussbaum said. Dosing begins at 5 milligrams and is then adjusted to the patient's needs. If a problem arises, doctors back off. It's classic trial and error.
But that may soon change. In the first major trial for dosing a drug based on a gene profile, physicians hope to find if they can predict the proper dosage based on a person's genes. Run by the University of Pennsylvania, pharmacists involved in the trial will determine the dosage based on the genetic profile, and neither doctors nor patients will know exactly how much is being administered.
Nussbaum says his group at UCSF plans to be among those medical groups whose patients are a part of the trial.
But as genetics develops new ways of diagnosing and treating patients, many are worried about whether laws and policies will keep pace.
Hamosh notes that she and genetic counselors in her clinic frequently have to fight to get insurance to pay for gene testing and some of the treatments that follow, which many insurance companies refuse to cover.
On the flip side, however, many are worrying about what will happen once all their genetic information is available, whether it will mean insurance companies denying medical coverage or the government using it to track everywhere they go.
Johns Hopkins' Chakravarti notes that genetic testing has many applications for preventive medicine that could ultimately save in health-care costs, but the only way to ensure that they are not used against a patient is through the courts.
"The only way in which we can prevent wide-scale discrimination base on someone's blueprint will be by legislation," he said.
But as Chakravarti points out, not all genetic information is harmful if released.
For example, he said, the first widespread genetic testing people underwent was for blood type in order to avoid problems when they underwent transfusions.
People are willing to tell other people their blood type without concern, he said, because it can be beneficial in many instances.
This, said Chakravarti, is the direction genetics research should head in future years -- tests with a clear benefit to the patient and with no negative uses.
"Ultimately, if there are genetic tests that are accurate and can be used for a well-defined medical need, we won't be afraid to share our medical information," he said.
But he notes that patients who worry how their DNA may be used have valid concerns, especially with the speed of technologies that are making sequencing easier and easier.
Governments may insist on collecting more information on citizens, he notes, and people will have to decide whether their motives are malicious or benign.
"You can't fake your DNA profile," he said. "Maybe that's your future identity."