Much of the $30 billion that Canadians spend on drugs every year is wasted, but personalized medicines could revolutionize the way doctors prescribe, a biopharmaceutical researcher says.

People have genetic differences that determine how their bodies react to drugs such as warfarin or Coumadin, a blood thinner taken to prevent clotting. It is tricky to get the dosing right, because too little has no effect but too much can cause bleeding.

"Maybe you need more or maybe you have too much," said Serge Carriere, a healthy 70-year-old in Montreal who takes warfarin because he is at high risk for stroke. "It depends on the individual, the way they metabolize the drug."

Dr. Jean-Claude Tardif of the Montreal Heart Institute is testing the use of genetic analysis to help tailor drug dosages for patients like Carriere.

"I think it has the potential to change how we use warfarin in the next couple of years," Tardif said.

Genetic discoveries increasingly mean doctors will be able to use a patient's genetic code to predict how they'll respond to different types of treatments.

Genetic tests are already used to identify people who may benefit from Herceptin for breast cancer, for example, or Gleevec for leukemia.

Researchers also know that some people's genetic makeup means antidepressants won't work for them, or that a drug commonly used to treat childhood leukemia will cause severe side-effects in some children.

"We know what the genetic defect is," said biopharmaceutical researcher Deanna Kroetz of the University of California, San Francisco. "And we know what to do about it. But there's still resistance from the physicians who treat these patients."

Even when predictive tests are available, many doctors won't use them, partly because they cost hundreds of dollars, but also because there's a lack of faith, comfort and confidence around genetics, Kroetz said.

"We need to teach them and tell them how to use this genetic information to better treat these patients," said Tardif.

It's a development that Carriere calls reassuring.