The world's largest retailer said it will make available for $4 drugs to treat glaucoma, attention deficit disorder/attention deficit hyperactivity disorder, fungal infections and acne. Fertility and prescription birth control will also be available for $9, Wal-Mart said.

Last year, Wal-Mart began selling certain generic drugs for $4 per monthly prescription in September and by the end of November had extended the program to all its U.S. pharmacies -- far ahead of schedule.

The company said $4 prescriptions now account for nearly 40 percent of all prescriptions filled in its Wal-Mart, Sam's Club and Neighborhood Market pharmacies. It estimates that over the past year, the program has saved customers $613.6 million.

Earlier this year, it said it would open as many as 400 in-store health clinics in the next two to three years, and that number could jump to 2,000 in five to seven years.