Supplies are scarce for 168 drugs -- compared to just 56 drugs reported scarce in 2006, according to the FDA.

Most of the affected drugs are those provided in a hospital setting -- including cancer drugs, anesthetics for those undergoing surgery, and electrolytes for IV feeding. The cancer drugs are among the worst hit, putting some half a million patients at risk.

Researchers at IMS Institute for Healthcare Informatics believe some drugmakers have just stopped making certain medications in recent years. One hundred companies supply the 168 products initially reported in short supply. Some of the scarce drugs are produced by only one or two companies, IMS executive director Murray Aitken said. With just a few suppliers, there's not a lot of flexibility should one have a manufacturing problem.

"Part of the story is there may not be sufficient economic incentive currently in this sector of the market," Aitken added.

The corporations supplying the most drugs on the shortage list were Hospira, Inc. and Teva Pharmaceuticals USA. Others included Novartis AG, Watson Pharmaceuticals, Pfizer, and Baxter Healthcare, researchers report.