DEERFIELD, Ill. (AP) -- Walgreen Co. will fill prescriptions at no upfront cost to some patients who don't have all the information they need for coverage received through the health care overhaul.

The nation's largest drugstore chain said Monday evening that it will provide up to a 30-day supply of some branded and generic medicines to patients who can confirm their enrollment through the overhaul's public insurance marketplaces but have yet to receive a plan identification number, which is normally needed to process a claim.

The Deerfield, Ill., company's offer excludes complex and expensive medications like cancer drugs or treatments for rare diseases.

These patients won't have to pay anything at the pharmacy, but they may receive a bill later for the co-payment their plan requires them to make. The drugstore chain also will collect payment from the insurer once it verifies enrollment.

The overhaul aims to cover millions of uninsured people, with many insurance customers signing up either through state-based insurance exchanges or a federally operated website, www.healthcare.gov . Website crashes and other problems marred the debut of that website last fall.

Insurers also are worried about technical problems that involve the government passing along inaccurate information on enrollees. It remains to be seen whether more complications will crop up when people try to use their benefits in the new year.

Walgreen shares closed at $57.84 on Monday. That price has advanced more than 56 percent so far this year.