For years, hype has built around personalized medicine - a tantalizing future in which insights gleaned from genetic tests will result in individualized treatment, guiding the drugs people take and at what doses. Now, moves by two large companies that focus on controlling drug costs are leading the way for the field to become a routine part of medicine.

CVS Caremark, the Woonsocket, R.I., company that is the largest provider of prescriptions in the United States, said late last year it expects to begin offering genetic testing services to clients of its pharmacy benefit management program this year. It also invested in Generation Health, a company with offices in Waltham that is focused on helping companies manage costs and improve health by using genetic information.

A CVS competitor, Medco Health Solutions, offers genetic tests to guide the use of two drugs and plans to add four more tests this year. Medco has 270 clients, representing 7 million people, participating in its personalized medicine program.

The companies work with insurance plans or large employers and use their buying power to keep drug costs low. They want to use genetic tests to sift out patients who are unlikely to benefit from a drug they have been prescribed or who could experience dangerous or costly side effects.

When a doctor submits a prescription, for example, the company that manages a patient’s drug benefits may call the doctor and offer a genetic test. Ultimately, results might discourage the doctor from prescribing a drug that won’t work, help determined what dosage to use, or suggest an equally effective generic option. CVS said that testing would be integrated into the prescription-filling process.

“The hope is that as we learn more and more about the genome . . . we’ll be in the situation where a lot of different kinds of medications will have the choice of the medication or the dosing of the medication indicated by a genetic test,’’ said Dr. Troyen Brennan, chief medical officer of CVS.

The companies’ interest in using genetic information stems from a longstanding problem: Drugs may be more effective in some patients than others, and doctors often have no way of knowing before they prescribe them. Insight from genetics is beginning to explain some of those differences.

Patients with a particular genetic makeup do not effectively break down Plavix, an anticlotting drug. Variations in two genes affect how people re spond to the common blood thinner warfarin. Genetic testing provides a new way to understand which drugs will work for which patients, meaning people and insurers might have a new tool to avoid paying for drugs that may not work.

“We’re in a good place, theoretically, because we’re being hired to help people manage their prescriptions,’’ said Dr. Robert Epstein, chief medical officer of Medco.