Why? A review of 29 studies - only three of which lasted more than 30 weeks - found that Merck’s Januvia and Lilly’s Byetta work better than placebos in controlling blood sugar, but are no more effective than drugs available for just a few pennies a pill. The research also showed the new drugs carry side effects older treatments don’t, such as nausea, vomiting and infections.

“It would be premature to fully embrace and use both these classes of medications over older, more-established medications for a chronic condition such as diabetes,” the lead researcher, Anastassios Pittas, an assistant professor of medicine at Tufts-New England Medical Center in Boston, told Bloomberg News, which reported this last night.

As the news service notes, docs have embraced the new drugs largely because they don’t cause low blood sugar and can lead to weight loss. But they are expensive: a month’s supply of 100-milligram tablets of Januvia costs $163.99, or almost $5.50 a pill, from Drugstore.com, while a single cartridge of the injected Byetta costs $200. Sixty tablets of metformin, a drug that makes the cells more sensitive to insulin, is $33.99.

“They are 10 to 12 times more expensive,” Pittas said. “The question is, are they 10 times better? It’s pretty amazing to me that we have six-month data and a lot of people are willing to jump on this, even though the drugs haven’t proven themselves yet and they are very, very expensive.”

The review was funded by the National Institute of Diabetes and Digestive and Kidney Diseases, and appears in the July 11 Journal of the American Medical Association.

In all, there are nine classes of diabetes drugs on the market. Byetta, approved in 2005, was the first of its class, known as a GLP-1 analogue. It is an injected synthetic hormone that stimulates cells in the pancreas to produce insulin. Januvia, approved last year, was the first in its category, known as DPP4 inhibitors.

The researchers said the headaches, urinary-tract infections and other ills seen with Januvia and Novartis AG’s Galvus were worrisome. The drugs block a protein that is expressed in many tissues, and use may affect immune-system function, the scientists said.

“Although the relative risk we measured was small, its implications in clinical practice are significant because there are more than 20 million patients with diabetes in the United States who are both more likely to develop a urinary tract infection and are at higher risk of complications, including death,” they wrote. “It may be prudent to avoid use of these agents in patients with a history of recurrent urinary tract infections” and headaches.

The drugmakers took exception. Januvia is effective for, and well tolerated by, patients with type 2 diabetes, the most-common form of the disease, said Amy Rose, a Merck spokeswoman, who added that almost 50 studies are complete or under way, and nine more will start this year, she said. About 4,700 patients have taken Januvia in the company’s research programs.

Numerous studies have established Byetta’s ability to control blood sugar in patients with type 2 diabetes, said Kindra Strupp, a Lilly spokeswoman. A study presented at the American Diabetes Association meeting last month showed it is effective and leads to progressive weight loss for three years, the longest study of the drug to date, she said.

“Byetta is an appropriate option to consider when people with type 2 diabetes cannot control their blood sugar using one or more common oral medications and before insulin is initiated,” she said. “Since approval Byetta has had an impact on over 600,000 people with type 2 diabetes and shown that therapies that lower blood glucose and allow most people to lose weight are a valuable new direction.”