Patent expirations, insurers and the economy are helping generic prescription drugs increase their share of the market.

Researchers at Wolters Kluwer Health, Bridgewater, N.J., come to that conclusion in their latest annual analysis of the U.S. pharmaceutical market.

Generic drugs will account for about two-thirds of all U.S. prescriptions filled by the end of the year, the researchers predict.

Since 2004, generic drug sales have increased 12% per year, while brand-name drug sales have fallen 6% per year, the researchers report.

In some cases, generic drug sales are increasing because popular drugs have come off patent, but, in other cases, financial pressure is leading patients to choose generics, the researchers write.

The researchers also see other signs of financial pressure affecting the U.S. drug market:

- In 2008, the percentage of patients who abandoned prescriptions reached 6.8%, up from about 5.1% in 2006.

- Prescription abandonment rates increased as patient co-payments increased. The abandonment rate for new prescriptions is 4% for prescriptions with a $10 co-pay and more than 20% for prescriptions with a $100 co-pay.