Federal regulators on Tuesday approved a new class of oral drugs for Type 2 diabetes that are as effective as most existing treatments and avoid common side effects, such as dangerously low blood sugar.

The Food and Drug Administration said that Merck & Co.’s Januvia, a once-daily tablet, could be taken alone or in combination with the commonly used oral diabetes medicines metformin, Actos and Avandia.

Dr. Robert Meyer of the FDA’s Office of Drug Evaluation called Januvia a “useful addition” to the treatment arsenal.

“Not everybody optimally responds to each medication, or not everybody can tolerate each medication,” he said at a news conference. “Having a new drug in a new class for such a widely prevalent disease is important.”

But Dr. John Buse, director of the diabetes center at the University of North Carolina School of Medicine and president-elect of the American Diabetes Association, said it was still uncertain how valuable the drug would be.

Some commonly used older drugs “are very effective and are ridiculously cheap,” said Buse, who has consulted for many pharmaceutical companies. “The only sin they committed was becoming generic” and losing corporate marketing support.

Januvia will cost $4.86 a day, adding about $1,800 to the annual cost of treatment.

The drug is among several medicines recently approved for Type 2 diabetes, an illness linked to obesity that is reaching epidemic proportions worldwide and affects 20 million people in the U.S.

Annual worldwide sales of diabetes drugs are expected to double to $20 billion over the next five years as more people develop the disease and pharmaceutical companies continue to bring out new and more expensive treatments.

Galvus, a Norvatis drug similar to Januvia, is expected to receive FDA approval next month.

People with Type 2 diabetes progressively lose their ability to produce or properly use insulin, a hormone made in the pancreas that moves sugar from the blood into cells, where it is used as fuel. A buildup of sugar in the blood not only starves cells of energy but can cause serious health complications including blindness, kidney failure and amputations.

Existing drugs act to lower blood-sugar levels in one of two ways. Sulfonylurea drugs stimulate the pancreas to release insulin, while metformin works to discourage the liver from producing sugar. Patients usually start on metformin and later add a sulfonylurea or another drug, but over time the medicines lose their effectiveness and eventually many patients need insulin shots.

Januvia and Galvus work differently. They raise the level of a hormone called GLP-1, which is produced in the intestines and stomach when blood sugar levels are elevated. The hormone in turn stimulates the pancreas to release insulin while simultaneously restraining the liver from making sugar.

The most common side effects of the new drugs, known as gliptins, include headache and a stuffy nose. They do not appear to cause weight gain, a problem with taking sulfonylureas or another class of diabetes drugs known as thiazolidinediones, which includes Actos and Avandia. The latter drugs carry a small risk of congestive heart failure.

“We think this will change the landscape for how physicians manage the disease,” Merck marketing vice president Patrick Magri said.

Most doctors and analysts do not think the gliptins will dislodge metformin, a more effective blood-sugar-lowering agent, as a first treatment for Type 2 diabetes. But some think the new medicines could eventually replace sulfonylureas as second-line drugs because they avoid the weight gain side effect.