Rite Aid is giving patients a chance to peek over their doctor's shoulder with genetic tests that help determine the effectiveness of some prescriptions.

The drugstore chain said Thursday it is selling Harmonyx testing kits at nearly all of its stores.

The kits cost between $49 and $89 without a prescription, and customers can use them to learn more about the effectiveness of medicines for cardiac conditions, cholesterol and attention deficit hyperactivity disorder.

The kits delve into a growing area of medical care, with doctors using genetic analysis more in specialties such as cardiology and oncology to make sure what they prescribe is a good fit for their patients.

The kits also broaden the array of self-administered patient tests drugstores have offered for years, a portfolio that can include tests for pregnancy, drugs and paternity.

But tests that explore a drug's effectiveness are more complex than those that just detect pregnancy hormones.

Genetic counselor Joy Larsen Haidle said patients should talk to their doctors before buying a test kit.

"If your doctor feels you should be on a medication, he should be part of the decision-making process," said Larsen Haidle, president of the National Society of Genetic Counselors. "Your doctor ordered the medicine for a reason and a specific dose."

She also recommends all genetic test customers find out before buying a test what will happen with their leftover sample after a company does the analysis. It may be discarded, or it might be sold for research.

Harmonyx destroys its samples after testing and doesn't share the results with a third party, Rite Aid spokeswoman Ashley Flower said.

Customers who buy the kits swab their cheeks to get a genetic sample and send it to Harmonyx. The company then delivers results to the patient, his or her physician and the drugstore pharmacist.

Genetic testing can tell whether a patient might metabolize a medicine quickly and needs a dose adjustment.

In cancer treatment, it also can offer insight into which drugs may be most effective against a particular tumor.

These tests can yield good information, Larsen Haidle said.

But she noted testing done through a doctor will be covered by health insurance, while patients pay the full price of a kit bought at a drugstore without a prescription.

Rite Aid Corp., which is being acquired by rival drugstore chain Walgreens, decided to sell the Harmonyx tests as part of its broader push to help customers "live and stay well," Flower said in an email.

That reflects a trend in the industry.

Drugstores have been focusing more on customer health for years, in part to appeal to aging baby boomers and the growing number of people who are shopping around more for health care instead of simply visiting their family doctors.